diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 0517c160b6..45b82eb733 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -1379,6 +1379,7 @@ nvim_replace_termcodes({str}, {from_part}, {do_lt}, {special})
       • Lua can use |vim.keycode()| instead.
 
     Attributes: ~
+        |api-fast|
         Since: 0.1.0
 
     Parameters: ~
diff --git a/runtime/doc/lua.txt b/runtime/doc/lua.txt
index c183a7e0cd..22b1e9e416 100644
--- a/runtime/doc/lua.txt
+++ b/runtime/doc/lua.txt
@@ -1273,7 +1273,7 @@ vim.inspect()                                                  *vim.inspect()*
 
 vim.keycode({keys}, {info})                                    *vim.keycode()*
     Converts keys from |key-notation| to the internal encoding. Optionally
-    returns structured key-chord info as retval 2.
+    returns structured *key-chord* info as retval 2.
 
     Inverse of |keytrans()|, which converts the internal encoding back to
     |key-notation|.
@@ -1347,33 +1347,36 @@ vim.notify_once({msg}, {level}, {opts})                    *vim.notify_once()*
         (`boolean`) true if message was displayed, else false
 
 vim.on_key({fn}, {ns_id}, {opts})                               *vim.on_key()*
-    Adds Lua function {fn} with namespace id {ns_id} as a listener to every,
-    yes every, input key.
+    Registers function {fn} with |namespace| {ns_id} as a listener to every,
+    yes EVERY, input key.
 
-    The Nvim command-line option |-w| is related but does not support
-    callbacks and cannot be toggled dynamically.
+    To parse |key-chord|s, see |vim.keycode()|. Example: >lua
+        local keychords = vim.keycode(vim.fn.keytrans(key), true)
+<
+
+    The |-w| command-line option is related but does not support callbacks and
+    cannot be toggled dynamically.
 
     Note: ~
+      • If {fn} returns an empty string, {key} is discarded/ignored; if {key}
+        is <Cmd> then the "<Cmd>…<CR>" sequence is discarded as a whole.
+      • Non-recursive: if {fn} itself consumes input, it won't be invoked for
+        those keys.
+      • To UNregister a given {ns_id}, pass `nil` {fn}.
       • {fn} will be removed on error.
-      • {fn} won't be invoked recursively, i.e. if {fn} itself consumes input,
-        it won't be invoked for those keys.
       • {fn} will not be cleared by |nvim_buf_clear_namespace()|
 
     Parameters: ~
       • {fn}     (`fun(key: string, typed: string): string??`) Function
                  invoked for every input key, after mappings have been applied
                  but before further processing. Arguments {key} and {typed}
-                 are raw keycodes, where {key} is the key after mappings are
-                 applied, and {typed} is the key(s) before mappings are
-                 applied. {typed} may be empty if {key} is produced by
-                 non-typed key(s) or by the same typed key(s) that produced a
-                 previous {key}. If {fn} returns an empty string, {key} is
-                 discarded/ignored, and if {key} is <Cmd> then the
-                 "<Cmd>…<CR>" sequence is discarded as a whole. When {fn} is
-                 `nil`, the callback associated with namespace {ns_id} is
-                 removed.
-      • {ns_id}  (`integer?`) Namespace ID. If nil or 0, generates and returns
-                 a new |nvim_create_namespace()| id.
+                 are raw input (use |keytrans()| to get |keycodes|).
+                 • {key} is the key after mappings are applied.
+                 • {typed} is the input before mappings are applied; may be
+                   empty if {key} was produced by non-typed key(s) or by the
+                   same typed key(s) that produced a previous {key}.
+      • {ns_id}  (`integer?`) Namespace ID. If nil or 0, returns a new
+                 |namespace| id.
       • {opts}   (`table?`) Optional parameters
 
     Return: ~
@@ -1382,6 +1385,7 @@ vim.on_key({fn}, {ns_id}, {opts})                               *vim.on_key()*
 
     See also: ~
       • |keytrans()|
+      • |vim.keycode()|
 
 vim.paste({lines}, {phase})                                      *vim.paste()*
     Paste handler, invoked by |nvim_paste()|.
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 1259391921..4e5ba1b508 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -386,6 +386,11 @@ VIMSCRIPT
 • |v:starttime| is the process start time (nanoseconds since UNIX epoch).
 • |v:useractive| indicates user activity.
 • |serverlist()| with `info=true` returns details for each server (own + peers).
+• These Vimscript functions may be called from |api-fast| context (e.g.
+  |vim.uv| timer/handle callbacks): |keytrans()|, |char2nr()|, |nr2char()|,
+  |str2list()|, |strpart()|, |strcharpart()|, |strgetchar()|, |strchars()|,
+  |strcharlen()|, |strlen()|, |strdisplaywidth()|, |byteidx()|, |charidx()|,
+  |utf16idx()|, |tr()|, |trim()|.
 
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
diff --git a/runtime/doc/vimfn.txt b/runtime/doc/vimfn.txt
index c0915ee20d..722ffe8a8c 100644
--- a/runtime/doc/vimfn.txt
+++ b/runtime/doc/vimfn.txt
@@ -99,6 +99,9 @@ api_info()                                                          *api_info()*
 		       lua vim.print(vim.fn.api_info())
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Return: ~
                   (`table`)
 
@@ -856,6 +859,9 @@ byteidx({expr}, {nr} [, {utf16}])                                    *byteidx()*
 			echo byteidx('a😊😊', 3, 1)	" returns 5
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`any`)
                   • {nr} (`integer`)
@@ -878,6 +884,9 @@ byteidxcomp({expr}, {nr} [, {utf16}])                            *byteidxcomp()*
 		character is 3 bytes), the second echo results in 1 ('e' is
 		one byte).
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`any`)
                   • {nr} (`integer`)
@@ -999,6 +1008,9 @@ char2nr({string} [, {utf8}])                                         *char2nr()*
 
 		Returns 0 if {string} is not a |String|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {utf8} (`any?`)
@@ -1083,6 +1095,9 @@ charidx({string}, {idx} [, {countcc} [, {utf16}]])                   *charidx()*
 			echo charidx('a😊😊', 4, 0, 1)	" returns 2
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {idx} (`integer`)
@@ -1731,6 +1746,9 @@ did_filetype()                                                  *did_filetype()*
 		editing another buffer to set 'filetype' and load a syntax
 		file.
 
+                Attributes: ~
+                  |api-fast|
+
                 Return: ~
                   (`integer`)
 
@@ -1887,6 +1905,9 @@ environ()                                                            *environ()*
 			echo index(keys(environ()), 'HOME', 0, 1) != -1
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Return: ~
                   (`any`)
 
@@ -1898,6 +1919,9 @@ escape({string}, {chars})                                             *escape()*
 			c:\\program\ files\\vim
 <		Also see |shellescape()| and |fnameescape()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {chars} (`string`)
@@ -1961,6 +1985,9 @@ executable({expr})                                                *executable()*
 			0	does not exist
 		|exepath()| can be used to get the full path of an executable.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`string`)
 
@@ -2010,6 +2037,9 @@ exepath({expr})                                                      *exepath()*
 		Returns empty string otherwise.
 		If {expr} starts with "./" the |current-directory| is used.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`string`)
 
@@ -2424,6 +2454,9 @@ filereadable({file})                                            *filereadable()*
 			1
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {file} (`string`)
 
@@ -2439,6 +2472,9 @@ filewritable({file})                                            *filewritable()*
 		is not writable, the result is 0.  If {file} is a
 		directory, and we can write to it, the result is 2.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {file} (`string`)
 
@@ -2696,6 +2732,9 @@ fnameescape({filepath})                                          *fnameescape()*
 			edit \+some\ str\%nge\|name
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {filepath} (`string`)
 
@@ -2723,6 +2762,9 @@ fnamemodify({fname}, {mods})                                     *fnamemodify()*
 		Note: Environment variables don't work in {fname}, use
 		|expand()| first then.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {fname} (`string`)
                   • {mods} (`string`)
@@ -3779,6 +3821,9 @@ getfperm({fname})                                                   *getfperm()*
 
 		For setting permissions use |setfperm()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {fname} (`string`)
 
@@ -3794,6 +3839,9 @@ getfsize({fname})                                                   *getfsize()*
 		If the size of {fname} is too big to fit in a Number then -2
 		is returned.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {fname} (`string`)
 
@@ -3809,6 +3857,9 @@ getftime({fname})                                                   *getftime()*
 		|localtime()| and |strftime()|.
 		If the file {fname} can't be found -1 is returned.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {fname} (`string`)
 
@@ -3837,6 +3888,9 @@ getftype({fname})                                                   *getftype()*
 		systems that support it.  On some systems only "dir" and
 		"file" are returned.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {fname} (`string`)
 
@@ -4051,6 +4105,9 @@ getpid()                                                              *getpid()*
 		Return a Number which is the process ID of the Vim process.
 		This is a unique number, until Vim exits.
 
+                Attributes: ~
+                  |api-fast|
+
                 Return: ~
                   (`integer`)
 
@@ -4908,6 +4965,9 @@ has({feature})                                                           *has()*
 			endif
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {feature} (`string`)
 
@@ -5135,6 +5195,9 @@ hostname()                                                          *hostname()*
 		Returns the hostname of the machine on which the Nvim server
 		(not the UI client) is currently running.
 
+                Attributes: ~
+                  |api-fast|
+
                 Return: ~
                   (`string`)
 
@@ -5152,6 +5215,9 @@ iconv({string}, {from}, {to})                                          *iconv()*
 		from/to UCS-2 is automatically changed to use UTF-8.  You
 		cannot use UCS-2 in a string anyway, because of the NUL bytes.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {from} (`string`)
@@ -5562,6 +5628,9 @@ isabsolutepath({path})                                        *isabsolutepath()*
 			echo isabsolutepath('\\remote\file')	" 1
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {path} (`string`)
 
@@ -5575,6 +5644,9 @@ isdirectory({directory})                                         *isdirectory()*
 		exist or isn't a directory.  {directory} is any expression,
 		which is used as a String.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {directory} (`string`)
 
@@ -5924,6 +5996,9 @@ keytrans({string})                                                  *keytrans()*
 			echo keytrans(xx)
 <			<C-Home>
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
 
@@ -7408,6 +7483,9 @@ nr2char({expr} [, {utf8}])                                           *nr2char()*
 		characters.  nr2char(0) is a real NUL and terminates the
 		string, thus results in an empty string.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`integer`)
                   • {utf8} (`boolean?`)
@@ -8310,6 +8388,9 @@ reltime({start}, {end})
 
 		Note: |localtime()| returns the current (non-relative) time.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {start} (`any?`)
                   • {end} (`any?`)
@@ -8331,6 +8412,9 @@ reltimefloat({time})                                            *reltimefloat()*
 		Also see |profiling|.
 		If there is an error an empty string is returned
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {time} (`any`)
 
@@ -8351,6 +8435,9 @@ reltimestr({time})                                                *reltimestr()*
 <		Also see |profiling|.
 		If there is an error an empty string is returned
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {time} (`any`)
 
@@ -8452,6 +8539,9 @@ repeat({expr}, {count})                                               *repeat()*
 			let longlist = repeat(['a', 'b'], 3)
 <		Results in ['a', 'b', 'a', 'b', 'a', 'b'].
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`any`)
                   • {count} (`integer`)
@@ -8472,6 +8562,9 @@ resolve({filename})                                             *resolve()* *E65
 		current directory (provided the result is still a relative
 		path name) and also keeps a trailing path separator.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {filename} (`string`)
 
@@ -10780,6 +10873,9 @@ state([{what}])                                                        *state()*
 			recursiveness up to "ccc")
 		    s	screen has scrolled for messages
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {what} (`string?`)
 
@@ -10837,6 +10933,9 @@ stdpath({what})                                                *stdpath()* *E610
 			echo stdpath("config")
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {what}
                     (`'cache'|'config'|'config_dirs'|'data'|'data_dirs'|'log'|'run'|'state'`)
@@ -10886,6 +10985,9 @@ str2list({string} [, {utf8}])                                       *str2list()*
 			echo str2list("á")		" returns [97, 769]
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {utf8} (`boolean?`)
@@ -10931,6 +11033,9 @@ strcharlen({string})                                              *strcharlen()*
 
 		Also see |strlen()|, |strdisplaywidth()| and |strwidth()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
 
@@ -10952,6 +11057,9 @@ strcharpart({src}, {start} [, {len} [, {skipcc}]])               *strcharpart()*
 
 		Returns an empty string on error.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {src} (`string`)
                   • {start} (`integer`)
@@ -10992,6 +11100,9 @@ strchars({string} [, {skipcc}])                                     *strchars()*
 		    endif
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {skipcc} (`0|1|boolean?`)
@@ -11019,6 +11130,9 @@ strdisplaywidth({string} [, {col}])                          *strdisplaywidth()*
 
 		Also see |strlen()|, |strwidth()| and |strchars()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {col} (`integer?`)
@@ -11061,6 +11175,9 @@ strgetchar({str}, {index})                                        *strgetchar()*
 		Returns -1 if {index} is invalid.
 		Also see |strcharpart()| and |strchars()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {str} (`string`)
                   • {index} (`integer`)
@@ -11091,6 +11208,9 @@ stridx({haystack}, {needle} [, {start}])                              *stridx()*
 		stridx() works similar to the C function strstr().  When used
 		with a single character it works similar to strchr().
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {haystack} (`string`)
                   • {needle} (`string`)
@@ -11145,6 +11265,9 @@ strlen({string})                                                      *strlen()*
 		|strchars()|.
 		Also see |len()|, |strdisplaywidth()| and |strwidth()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
 
@@ -11180,6 +11303,9 @@ strpart({src}, {start} [, {len} [, {chars}]])                        *strpart()*
 			strpart(getline("."), col(".") - 1, 1, v:true)
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {src} (`string`)
                   • {start} (`integer`)
@@ -11258,6 +11384,9 @@ strtrans({string})                                                  *strtrans()*
 
 		Returns an empty string on error.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
 
@@ -11306,6 +11435,9 @@ strwidth({string})                                                  *strwidth()*
 		Returns zero on error.
 		Also see |strlen()|, |strdisplaywidth()| and |strchars()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
 
@@ -12004,6 +12136,9 @@ tolower({expr})                                                      *tolower()*
 		Converts a String to lowercase (like applying |gu|).
 		Returns empty string on error.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`string`)
 
@@ -12016,6 +12151,9 @@ toupper({expr})                                                      *toupper()*
 		Converts a String to uppercase (like applying |gU|).
 		Returns empty string on error.
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`string`)
 
@@ -12036,6 +12174,9 @@ tr({src}, {fromstr}, {tostr})                                             *tr()*
 			echo tr("<blob>", "<>", "{}")
 <		returns "{blob}"
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {src} (`string`)
                   • {fromstr} (`string`)
@@ -12074,6 +12215,9 @@ trim({text} [, {mask} [, {dir}]])                                       *trim()*
 			echo trim("  vim  ", " ", 2)
 <		returns "  vim"
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {text} (`string`)
                   • {mask} (`string?`)
@@ -12134,6 +12278,9 @@ type({expr})                                                            *type()*
 			if exists('v:t_number') | endif
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {expr} (`any`)
 
@@ -12261,6 +12408,9 @@ utf16idx({string}, {idx} [, {countcc} [, {charidx}]])               *utf16idx()*
 			echo utf16idx('a😊😊', 9)	" returns -1
 <
 
+                Attributes: ~
+                  |api-fast|
+
                 Parameters: ~
                   • {string} (`string`)
                   • {idx} (`integer`)
@@ -12697,6 +12847,9 @@ windowsversion()                                              *windowsversion()*
 
 		Lua: see |uv.os_uname()|.
 
+                Attributes: ~
+                  |api-fast|
+
                 Return: ~
                   (`string`)
 
diff --git a/runtime/lua/vim/_core/editor.lua b/runtime/lua/vim/_core/editor.lua
index 8e1971402f..98cd7b5ad6 100644
--- a/runtime/lua/vim/_core/editor.lua
+++ b/runtime/lua/vim/_core/editor.lua
@@ -713,31 +713,34 @@ end
 
 local on_key_cbs = {} --- @type table<integer,[function, table]>
 
---- Adds Lua function {fn} with namespace id {ns_id} as a listener to every,
---- yes every, input key.
+--- Registers function {fn} with [namespace] {ns_id} as a listener to every, yes EVERY, input key.
 ---
---- The Nvim command-line option |-w| is related but does not support callbacks
---- and cannot be toggled dynamically.
+--- To parse [key-chord]s, see |vim.keycode()|. Example:
+--- ```lua
+--- local keychords = vim.keycode(vim.fn.keytrans(key), true)
+--- ```
 ---
+--- The |-w| command-line option is related but does not support callbacks and cannot be toggled
+--- dynamically.
+---
+---@note If {fn} returns an empty string, {key} is discarded/ignored; if {key} is [<Cmd>] then the
+---      "[<Cmd>]…[<CR>]" sequence is discarded as a whole.
+---@note Non-recursive: if {fn} itself consumes input, it won't be invoked for those keys.
+---@note To UNregister a given {ns_id}, pass `nil` {fn}.
 ---@note {fn} will be removed on error.
----@note {fn} won't be invoked recursively, i.e. if {fn} itself consumes input,
----           it won't be invoked for those keys.
 ---@note {fn} will not be cleared by |nvim_buf_clear_namespace()|
 ---
 ---@param fn nil|fun(key: string, typed: string): string? Function invoked for every input key,
----          after mappings have been applied but before further processing. Arguments
----          {key} and {typed} are raw keycodes, where {key} is the key after mappings
----          are applied, and {typed} is the key(s) before mappings are applied.
----          {typed} may be empty if {key} is produced by non-typed key(s) or by the
----          same typed key(s) that produced a previous {key}.
----          If {fn} returns an empty string, {key} is discarded/ignored, and if {key}
----          is [<Cmd>] then the "[<Cmd>]…[<CR>]" sequence is discarded as a whole.
----          When {fn} is `nil`, the callback associated with namespace {ns_id} is removed.
----@param ns_id integer? Namespace ID. If nil or 0, generates and returns a
----                      new |nvim_create_namespace()| id.
+---          after mappings have been applied but before further processing. Arguments {key} and
+---          {typed} are raw input (use [keytrans()] to get [keycodes]).
+---          - {key} is the key after mappings are applied.
+---          - {typed} is the input before mappings are applied; may be empty if {key} was produced
+---            by non-typed key(s) or by the same typed key(s) that produced a previous {key}.
+---@param ns_id integer? Namespace ID. If nil or 0, returns a new |namespace| id.
 ---@param opts table? Optional parameters
 ---
 ---@see |keytrans()|
+---@see |vim.keycode()|
 ---
 ---@return integer Namespace id associated with {fn}. Or count of all callbacks
 ---if on_key() is called without arguments.
@@ -1277,8 +1280,8 @@ end
 --- A list of single character modifiers of the key.
 --- @field mod ('M'|'T'|'C'|'S'|'2'|'3'|'4'|'D')[]
 
---- Converts keys from [key-notation] to the internal encoding. Optionally returns
---- structured key-chord info as retval 2.
+--- Converts keys from [key-notation] to the internal encoding. Optionally returns structured
+--- [key-chord]() info as retval 2.
 ---
 --- Inverse of [keytrans()], which converts the internal encoding back to [key-notation].
 ---
diff --git a/src/gen/gen_eval_files.lua b/src/gen/gen_eval_files.lua
index cd6df9a891..76f450b8c6 100755
--- a/src/gen/gen_eval_files.lua
+++ b/src/gen/gen_eval_files.lua
@@ -571,6 +571,12 @@ local function render_eval_doc(f, fun, write)
     write('')
   end
 
+  if fun.fast then
+    write(util.md_to_vimdoc('Attributes: ~', 16, 16, TEXT_WIDTH))
+    write(util.md_to_vimdoc('|api-fast|', 18, 18, TEXT_WIDTH))
+    write('')
+  end
+
   if #params > 0 then
     write(util.md_to_vimdoc('Parameters: ~', 16, 16, TEXT_WIDTH))
     for i, param in ipairs(params) do
diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 6b5ed59e07..389cc379b5 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -493,7 +493,7 @@ error:
 /// @param do_lt      Also translate [<lt>]. Ignored if `special` is false.
 /// @param special    Replace |keycodes|, e.g. [<CR>] becomes a "\r" char.
 String nvim_replace_termcodes(String str, Boolean from_part, Boolean do_lt, Boolean special)
-  FUNC_API_SINCE(1) FUNC_API_RET_ALLOC
+  FUNC_API_SINCE(1) FUNC_API_FAST FUNC_API_RET_ALLOC
 {
   if (str.size == 0) {
     // Empty string
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index a07e3be0c3..f966f48da4 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -1233,6 +1233,7 @@ M.funcs = {
       	echo charidx('a😊😊', 4, 0, 1)	" returns 2
       <
     ]=],
+    fast = true,
     name = 'charidx',
     params = {
       { 'string', 'string' },
@@ -6506,6 +6507,7 @@ M.funcs = {
       <	<C-Home>
 
     ]=],
+    fast = true,
     name = 'keytrans',
     params = { { 'string', 'string' } },
     returns = 'string',
@@ -8099,6 +8101,7 @@ M.funcs = {
       string, thus results in an empty string.
 
     ]=],
+    fast = true,
     name = 'nr2char',
     params = { { 'expr', 'integer' }, { 'utf8', 'boolean' } },
     returns = 'string',
@@ -11818,6 +11821,7 @@ M.funcs = {
       	echo str2list("á")		" returns [97, 769]
       <
     ]=],
+    fast = true,
     name = 'str2list',
     params = { { 'string', 'string' }, { 'utf8', 'boolean' } },
     signature = 'str2list({string} [, {utf8}])',
@@ -11865,6 +11869,7 @@ M.funcs = {
       Also see |strlen()|, |strdisplaywidth()| and |strwidth()|.
 
     ]=],
+    fast = true,
     name = 'strcharlen',
     params = { { 'string', 'string' } },
     returns = 'integer',
@@ -11931,6 +11936,7 @@ M.funcs = {
           endif
       <
     ]=],
+    fast = true,
     name = 'strchars',
     params = { { 'string', 'string' }, { 'skipcc', '0|1|boolean' } },
     returns = 'integer',
@@ -11958,6 +11964,7 @@ M.funcs = {
       Also see |strlen()|, |strwidth()| and |strchars()|.
 
     ]=],
+    fast = true,
     name = 'strdisplaywidth',
     params = { { 'string', 'string' }, { 'col', 'integer' } },
     returns = 'integer',
@@ -12005,6 +12012,7 @@ M.funcs = {
       Also see |strcharpart()| and |strchars()|.
 
     ]=],
+    fast = true,
     name = 'strgetchar',
     params = { { 'str', 'string' }, { 'index', 'integer' } },
     returns = 'integer',
@@ -12091,6 +12099,7 @@ M.funcs = {
       Also see |len()|, |strdisplaywidth()| and |strwidth()|.
 
     ]=],
+    fast = true,
     name = 'strlen',
     params = { { 'string', 'string' } },
     returns = 'integer',
@@ -13085,6 +13094,7 @@ M.funcs = {
       <returns "{blob}"
 
     ]=],
+    fast = true,
     name = 'tr',
     params = { { 'src', 'string' }, { 'fromstr', 'string' }, { 'tostr', 'string' } },
     returns = 'string',
@@ -13122,6 +13132,7 @@ M.funcs = {
       <returns "  vim"
 
     ]=],
+    fast = true,
     name = 'trim',
     params = { { 'text', 'string' }, { 'mask', 'string' }, { 'dir', '0|1|2' } },
     returns = 'string',
@@ -13317,6 +13328,7 @@ M.funcs = {
       	echo utf16idx('a😊😊', 9)	" returns -1
       <
     ]=],
+    fast = true,
     name = 'utf16idx',
     params = {
       { 'string', 'string' },
diff --git a/src/nvim/input.c b/src/nvim/input.c
index 23ae750751..acea3ca1ac 100644
--- a/src/nvim/input.c
+++ b/src/nvim/input.c
@@ -1837,7 +1837,7 @@ int vgetc(void)
 
   // Execute Lua on_key callbacks.
   kvi_push(on_key_buf, NUL);
-  if (nlua_execute_on_key(c, on_key_buf.items)) {
+  if (nlua_exec_on_key(c, on_key_buf.items)) {
     // Keys following K_COMMAND/K_LUA/K_PASTE_START aren't normally received by
     // vim.on_key() callbacks, so discard them along with the current key.
     if (c == K_COMMAND) {
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 8f3589e5ef..c6bf2fe586 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -2217,7 +2217,7 @@ char *nlua_register_table_as_callable(const typval_T *const arg)
 }
 
 /// @return true to discard the key
-bool nlua_execute_on_key(int c, char *typed_buf)
+bool nlua_exec_on_key(int c, char *typed_buf)
 {
   static bool recursive = false;
 
diff --git a/test/functional/api/fast_spec.lua b/test/functional/api/fast_spec.lua
new file mode 100644
index 0000000000..507fbc4c2c
--- /dev/null
+++ b/test/functional/api/fast_spec.lua
@@ -0,0 +1,171 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local clear = n.clear
+local eq = t.eq
+local exec_lua = n.exec_lua
+
+describe('|api-fast| functions', function()
+  before_each(clear)
+
+  it('callable in a fast event', function()
+    local out = exec_lua(function()
+      local result
+      local timer = assert(vim.uv.new_timer())
+      timer:start(0, 0, function()
+        timer:close()
+        local ok
+        ok, result = pcall(function()
+          return {
+            byteidx = vim.fn.byteidx('héllo', 2),
+            char2nr = vim.fn.char2nr('A'),
+            charidx = vim.fn.charidx('héllo', 3),
+            in_fast = vim.in_fast_event(),
+            keycode = vim.keycode('<C-a>'),
+            keytrans = vim.fn.keytrans(vim.keycode('<C-Home>')),
+            nr2char = vim.fn.nr2char(65),
+            replace_termcodes = vim.api.nvim_replace_termcodes('<Esc>', true, true, true),
+            str2list = vim.fn.str2list('AB'),
+            strcharlen = vim.fn.strcharlen('abc'),
+            strcharpart = vim.fn.strcharpart('héllo', 1, 2),
+            strchars = vim.fn.strchars('abc'),
+            strdisplaywidth = vim.fn.strdisplaywidth('ab'),
+            strgetchar = vim.fn.strgetchar('abc', 1),
+            strlen = vim.fn.strlen('abc'),
+            strpart = vim.fn.strpart('abcdef', 1, 3),
+            strtrans = vim.fn.strtrans('a\001b'),
+            tr = vim.fn.tr('abc', 'ab', 'AB'),
+            trim = vim.fn.trim('  x  '),
+            utf16idx = vim.fn.utf16idx('abc', 2),
+          }
+        end)
+      end)
+      vim.wait(2000, function()
+        return result ~= nil
+      end)
+      return result
+    end)
+    eq({
+      byteidx = 3,
+      char2nr = 65,
+      charidx = 2,
+      in_fast = true,
+      keycode = '\1', -- <C-a>
+      keytrans = '<C-Home>',
+      nr2char = 'A',
+      replace_termcodes = '\27', -- <Esc>
+      str2list = { 65, 66 },
+      strcharlen = 3,
+      strcharpart = 'él',
+      strchars = 3,
+      strdisplaywidth = 2,
+      strgetchar = 98,
+      strlen = 3,
+      strpart = 'bcd',
+      strtrans = 'a^Ab',
+      tr = 'ABc',
+      trim = 'x',
+      utf16idx = 2,
+    }, out)
+  end)
+
+  it('do not trigger os_breakcheck()', function()
+    local res = exec_lua(function()
+      local polls = 0
+      -- os_breakcheck() polls the event loop (loop_poll_events), which fires a uv_check handle.
+      local chk = assert(vim.uv.new_check())
+      chk:start(function()
+        polls = polls + 1
+      end)
+      local function polled(fn)
+        polls = 0
+        fn()
+        return polls ~= 0
+      end
+
+      -- Big (multibyte) input => per-char breakcheck would exceed BREAKCHECK_SKIP (1000, or 100_000
+      -- for `veryfast_breakcheck`). Small input could pass (false negative).
+      local big = ('héllo wörld '):rep(100000) -- 1.2M chars.
+
+      local cases = {
+        byteidx = function()
+          vim.fn.byteidx(big, 500000)
+        end,
+        char2nr = function()
+          vim.fn.char2nr('A')
+        end,
+        charidx = function()
+          vim.fn.charidx(big, 500000)
+        end,
+        keycode = function()
+          vim.keycode('<C-a>')
+        end,
+        keytrans = function()
+          vim.fn.keytrans(vim.keycode('<C-Home>'))
+        end,
+        nr2char = function()
+          vim.fn.nr2char(65)
+        end,
+        replace_termcodes = function()
+          vim.api.nvim_replace_termcodes('<Esc>', true, true, true)
+        end,
+        str2list = function()
+          vim.fn.str2list(big)
+        end,
+        strcharlen = function()
+          vim.fn.strcharlen(big)
+        end,
+        strcharpart = function()
+          vim.fn.strcharpart(big, 1, 2)
+        end,
+        strchars = function()
+          vim.fn.strchars(big)
+        end,
+        strdisplaywidth = function()
+          vim.fn.strdisplaywidth(big)
+        end,
+        strgetchar = function()
+          vim.fn.strgetchar(big, 500000)
+        end,
+        strlen = function()
+          vim.fn.strlen(big)
+        end,
+        strpart = function()
+          vim.fn.strpart(big, 1, 3)
+        end,
+        strtrans = function()
+          vim.fn.strtrans(big)
+        end,
+        tr = function()
+          vim.fn.tr(big, 'ho', 'HO')
+        end,
+        trim = function()
+          vim.fn.trim(big)
+        end,
+        utf16idx = function()
+          vim.fn.utf16idx(big, 500000)
+        end,
+      }
+
+      local bad = {}
+      for name, fn in pairs(cases) do
+        if polled(fn) then
+          bad[#bad + 1] = name
+        end
+      end
+      table.sort(bad)
+
+      -- Positive control: a big Vimscript loop calls line_breakcheck() every iteration.
+      local control = polled(function()
+        vim.api.nvim_exec2('for i in range(1500000)|endfor', {})
+      end)
+
+      chk:stop()
+      chk:close()
+      return { bad = bad, control = control }
+    end)
+
+    eq(true, res.control) -- Confirm our detector actually works: breakcheck was seen.
+    eq({}, res.bad) -- No api-fast function polled the event-loop.
+  end)
+end)