diff --git a/runtime/doc/dev_arch.txt b/runtime/doc/dev_arch.txt
index dac2831ecb..7604497d84 100644
--- a/runtime/doc/dev_arch.txt
+++ b/runtime/doc/dev_arch.txt
@@ -112,6 +112,12 @@ Many of the editor concepts are defined as Lua data files:
 - Ex (cmdline) commands: src/nvim/ex_cmds.lua
 - Options: src/nvim/options.lua
 - Vimscript functions: src/nvim/eval.lua
+  - Functions can be implemented in C (`f_foo` in eval/funcs.c), or in Lua
+    (set `func_lua` in eval.lua, implement in _core/vimfn.lua). Lua impl is
+    preferred, for performance and maintainability.
+    - VimScript callers go through `lua_wrapper` (typval <=> Object conversion).
+    - Lua callers (`vim.fn.foo()`) take a fast path in `nlua_call()` that calls
+      the Lua function directly, avoiding any conversion.
 - v: variables: src/nvim/vvars.lua
 
 ==============================================================================
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index aa255ce9ed..6b30a8b9e3 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -158,7 +158,10 @@ OPTIONS
 
 PERFORMANCE
 
-• todo
+• Nvim architecture allows pure-Lua implementations of some `vim.fn`
+  functions, which skips the Vimscript <=> Lua "bridge" (no data
+  conversion/marshalling) entirely, if the `vim.fn` function is called from
+  Lua.
 
 PLUGINS
 
diff --git a/runtime/doc/vimfn.txt b/runtime/doc/vimfn.txt
index 85b96cc975..04830089d4 100644
--- a/runtime/doc/vimfn.txt
+++ b/runtime/doc/vimfn.txt
@@ -4943,8 +4943,7 @@ hlexists({name})                                                    *hlexists()*
 
 hostname()                                                          *hostname()*
 		Returns the hostname of the machine on which the Nvim server
-		(not the UI client) is currently running.  Names greater than
-		256 characters long are truncated.
+		(not the UI client) is currently running.
 
                 Return: ~
                   (`string`)
diff --git a/runtime/lua/vim/_core/vimfn.lua b/runtime/lua/vim/_core/vimfn.lua
new file mode 100644
index 0000000000..12f7da1e74
--- /dev/null
+++ b/runtime/lua/vim/_core/vimfn.lua
@@ -0,0 +1,14 @@
+-- Lua implementations of "vimfn" builtin functions (via `func_lua`).
+--
+-- Functions defined here are pure Lua, they don't have any explicit C impl, so they are named with
+-- the "f_xx" convention, for discoverability.
+
+local M = {}
+
+--- Returns the hostname of the machine.
+--- @return string
+function M.f_hostname()
+  return vim.uv.os_gethostname()
+end
+
+return M
diff --git a/runtime/lua/vim/_meta/vimfn.gen.lua b/runtime/lua/vim/_meta/vimfn.gen.lua
index 8c72d7f111..8d6053e71e 100644
--- a/runtime/lua/vim/_meta/vimfn.gen.lua
+++ b/runtime/lua/vim/_meta/vimfn.gen.lua
@@ -4475,8 +4475,7 @@ function vim.fn.hlID(name) end
 function vim.fn.hlexists(name) end
 
 --- Returns the hostname of the machine on which the Nvim server
---- (not the UI client) is currently running.  Names greater than
---- 256 characters long are truncated.
+--- (not the UI client) is currently running.
 ---
 --- @return string
 function vim.fn.hostname() end
diff --git a/src/gen/gen_eval.lua b/src/gen/gen_eval.lua
index 7ba60e9263..4489af7e85 100644
--- a/src/gen/gen_eval.lua
+++ b/src/gen/gen_eval.lua
@@ -54,10 +54,16 @@ hashpipe:write([[
 ]])
 
 local funcs = loadfile(eval_file)().funcs
-for _, func in pairs(funcs) do
-  if func.float_func then
+for name, func in pairs(funcs) do
+  local n = (func.func_float and 1 or 0) + (func.func_lua and 1 or 0) + (func.func and 1 or 0)
+  assert(n <= 1, name .. ': only one of func, func_float, func_lua can be set')
+
+  if func.func_float then
     func.func = 'float_op_wrapper'
-    func.data = '{ .float_func = &' .. func.float_func .. ' }'
+    func.data = '{ .func_float = &' .. func.func_float .. ' }'
+  elseif func.func_lua then
+    func.func = 'lua_wrapper'
+    func.data = '{ .func_lua = "' .. func.func_lua .. '" }'
   end
 end
 
@@ -67,7 +73,7 @@ for _, fun in ipairs(metadata) do
     funcs[fun.name] = {
       args = #fun.parameters,
       func = 'api_wrapper',
-      data = '{ .api_handler = &method_handlers[' .. fun.handler_id .. '] }',
+      data = '{ .func_api = &method_handlers[' .. fun.handler_id .. '] }',
     }
   end
 end
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 50a38feb4f..4d47fadbf1 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -1,18 +1,12 @@
--- File containing table with all functions.
---
--- Keys:
+-- Defines all "vimfn" (builtin/"eval"/"Vimscript") functions.
 --
 --- @class vim.EvalFn
 --- @field name? string
---- @field args? integer|integer[] Number of arguments, list with maximum and minimum number of arguments
----       or list with a minimum number of arguments only. Defaults to zero
----       arguments.
---- @field base? integer For methods: the argument to use as the base argument (1-indexed):
----       base->method()
----       Defaults to BASE_NONE (function cannot be used as a method).
---- @field func? string Name of the C function which implements the Vimscript function. Defaults to
----       `f_{funcname}`.
---- @field float_func? string
+--- @field args? integer|integer[] (default: 0) Number of arguments, list with maximum and minimum number of arguments or list with a minimum number of arguments only.
+--- @field base? integer For methods: the argument to use as the base argument (1-indexed): base->method(). Defaults to BASE_NONE (function cannot be used as a method).
+--- @field func? string (default: "f_{funcname}") C function which implements the vimfn.
+--- @field func_float? string Floating-point C function. Sets func="float_op_wrapper".
+--- @field func_lua? string Function name in `vim._core.vimfn.*` (e.g. "f_hostname"). Sets func="lua_wrapper".
 --- @field fast? boolean Function can run in |api-fast| events. Defaults to false.
 --- @field deprecated? true
 --- @field returns? string|false
@@ -76,7 +70,7 @@ M.funcs = {
       <	2.094395
 
     ]=],
-    float_func = 'acos',
+    func_float = 'acos',
     name = 'acos',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -276,7 +270,7 @@ M.funcs = {
       <	-0.523599
 
     ]=],
-    float_func = 'asin',
+    func_float = 'asin',
     name = 'asin',
     params = { { 'expr', 'any' } },
     returns = 'number',
@@ -567,7 +561,7 @@ M.funcs = {
       <	-1.326405
 
     ]=],
-    float_func = 'atan',
+    func_float = 'atan',
     name = 'atan',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -1022,7 +1016,7 @@ M.funcs = {
       Returns 0.0 if {expr} is not a |Float| or a |Number|.
 
     ]=],
-    float_func = 'ceil',
+    func_float = 'ceil',
     name = 'ceil',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -1597,7 +1591,7 @@ M.funcs = {
       <	-0.646043
 
     ]=],
-    float_func = 'cos',
+    func_float = 'cos',
     name = 'cos',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -1618,7 +1612,7 @@ M.funcs = {
       <	-1.127626
 
     ]=],
-    float_func = 'cosh',
+    func_float = 'cosh',
     name = 'cosh',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -2357,7 +2351,7 @@ M.funcs = {
       <	0.367879
 
     ]=],
-    float_func = 'exp',
+    func_float = 'exp',
     name = 'exp',
     params = { { 'expr', 'number' } },
     signature = 'exp({expr})',
@@ -2882,7 +2876,7 @@ M.funcs = {
       <	4.0
 
     ]=],
-    float_func = 'floor',
+    func_float = 'floor',
     name = 'floor',
     params = { { 'expr', 'number' } },
     signature = 'floor({expr})',
@@ -5515,10 +5509,10 @@ M.funcs = {
   hostname = {
     desc = [=[
       Returns the hostname of the machine on which the Nvim server
-      (not the UI client) is currently running.  Names greater than
-      256 characters long are truncated.
+      (not the UI client) is currently running.
     ]=],
     fast = true,
+    func_lua = 'f_hostname',
     name = 'hostname',
     params = {},
     returns = 'string',
@@ -6576,7 +6570,7 @@ M.funcs = {
       <	5.0
 
     ]=],
-    float_func = 'log',
+    func_float = 'log',
     name = 'log',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -6596,7 +6590,7 @@ M.funcs = {
       <	-2.0
 
     ]=],
-    float_func = 'log10',
+    func_float = 'log10',
     name = 'log10',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -9058,7 +9052,7 @@ M.funcs = {
       <	-5.0
 
     ]=],
-    float_func = 'round',
+    func_float = 'round',
     name = 'round',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -11047,7 +11041,7 @@ M.funcs = {
       <	0.763301
 
     ]=],
-    float_func = 'sin',
+    func_float = 'sin',
     name = 'sin',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -11068,7 +11062,7 @@ M.funcs = {
       <	-1.026517
 
     ]=],
-    float_func = 'sinh',
+    func_float = 'sinh',
     name = 'sinh',
     params = { { 'expr', 'number' } },
     signature = 'sinh({expr})',
@@ -11335,7 +11329,7 @@ M.funcs = {
       NaN may be different, it depends on system libraries.
 
     ]=],
-    float_func = 'sqrt',
+    func_float = 'sqrt',
     name = 'sqrt',
     params = { { 'expr', 'number' } },
     signature = 'sqrt({expr})',
@@ -12517,7 +12511,7 @@ M.funcs = {
       <	-1.181502
 
     ]=],
-    float_func = 'tan',
+    func_float = 'tan',
     name = 'tan',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -12538,7 +12532,7 @@ M.funcs = {
       <	-0.761594
 
     ]=],
-    float_func = 'tanh',
+    func_float = 'tanh',
     name = 'tanh',
     params = { { 'expr', 'number' } },
     returns = 'number',
@@ -12798,7 +12792,7 @@ M.funcs = {
       <	4.0
 
     ]=],
-    float_func = 'trunc',
+    func_float = 'trunc',
     name = 'trunc',
     params = { { 'expr', 'number' } },
     returns = 'integer',
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index bbf49ee2f3..a5101d1cb8 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -227,7 +227,7 @@ char *get_expr_name(expand_T *xp, int idx)
   return get_user_var_name(xp, ++intidx);
 }
 
-/// Find internal function in hash functions
+/// Gets a builtin (aka "vimfn", "eval") function from the generated hash table.
 ///
 /// @param[in]  name  Name of the function.
 ///
@@ -240,6 +240,17 @@ const EvalFuncDef *find_internal_func(const char *const name)
   return index >= 0 ? &functions[index] : NULL;
 }
 
+/// Gets the Lua name of a Lua-implemented "vimfn" function, or NULL if not found.
+const char *find_internal_func_lua(const char *const name)
+  FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_PURE FUNC_ATTR_NONNULL_ALL
+{
+  const EvalFuncDef *const fdef = find_internal_func(name);
+  if (fdef && fdef->func == &lua_wrapper) {
+    return fdef->data.func_lua;
+  }
+  return NULL;
+}
+
 /// Check the argument count to use for internal function "fdef".
 /// @return  -1 for failure, 0 if no method base accepted, 1 if method base is
 /// first argument, 2 if method base is second argument, etc.
@@ -325,7 +336,7 @@ static bool non_zero_arg(typval_T *argvars)
               && *argvars[0].vval.v_string != NUL));
 }
 
-/// Apply a floating point C function on a typval with one float_T.
+/// Apply a floating point C function on a typval with one float_T (`func_float` in eval.lua).
 ///
 /// Some versions of glibc on i386 have an optimization that makes it harder to
 /// call math functions indirectly from inside an inlined function, causing
@@ -336,19 +347,23 @@ static void float_op_wrapper(typval_T *argvars, typval_T *rettv, EvalFuncData fp
 
   rettv->v_type = VAR_FLOAT;
   if (tv_get_float_chk(argvars, &f)) {
-    rettv->vval.v_float = fptr.float_func(f);
+    rettv->vval.v_float = fptr.func_float(f);
   } else {
     rettv->vval.v_float = 0.0;
   }
 }
 
+/// Invokes an API (nvim_) function from Vimscript.
+///
+/// Converts `argvars` to API Objects, calls the API handler, and converts the result back.
+/// Used by `gen_eval.lua` for `eval=true` API functions.
 static void api_wrapper(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
   if (check_secure()) {
     return;
   }
 
-  MsgpackRpcRequestHandler handler = *fptr.api_handler;
+  MsgpackRpcRequestHandler handler = *fptr.func_api;
 
   MAXSIZE_TEMP_ARRAY(args, MAX_FUNC_ARGS);
   Arena arena = ARENA_EMPTY;
@@ -375,11 +390,43 @@ end:
   api_clear_error(&err);
 }
 
+/// Invokes a Lua-implemented vimfn/"f_xx" function from Vimscript (`func_lua` in eval.lua).
+///
+/// - Converts argvars to API Objects, calls the Lua function, converts the result back.
+/// - NOT used when called from Lua; `nlua_call()` calls the function directly.
+static void lua_wrapper(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+{
+  MAXSIZE_TEMP_ARRAY(args, MAX_FUNC_ARGS);
+  Arena arena = ARENA_EMPTY;
+
+  for (typval_T *tv = argvars; tv->v_type != VAR_UNKNOWN; tv++) {
+    ADD_C(args, vim_to_object(tv, &arena, false));
+  }
+
+  // `func_lua` is the function name (e.g. "f_hostname") in `_core/vimfn.lua`.
+  char buf[256];
+  snprintf(buf, sizeof(buf), "return require('vim._core.vimfn').%s(...)", fptr.func_lua);
+
+  Error err = ERROR_INIT;
+  Object result = nlua_exec(cstr_as_string(buf), NULL, args, kRetObject, &arena, &err);
+
+  if (ERROR_SET(&err)) {
+    semsg_multiline("emsg", e_api_error, err.msg);
+    goto end;
+  }
+
+  object_to_vim_take_luaref(&result, rettv, true, &err);
+
+end:
+  arena_mem_free(arena_finish(&arena));
+  api_clear_error(&err);
+}
+
 /// "abs(expr)" function
 static void f_abs(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
   if (argvars[0].v_type == VAR_FLOAT) {
-    float_op_wrapper(argvars, rettv, (EvalFuncData){ .float_func = &fabs });
+    float_op_wrapper(argvars, rettv, (EvalFuncData){ .func_float = &fabs });
   } else {
     bool error = false;
 
@@ -2901,16 +2948,6 @@ static void f_hlexists(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   rettv->vval.v_number = highlight_exists(tv_get_string(&argvars[0]));
 }
 
-/// "hostname()" function
-static void f_hostname(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
-{
-  char hostname[256];
-
-  os_get_hostname(hostname, 256);
-  rettv->v_type = VAR_STRING;
-  rettv->vval.v_string = xstrdup(hostname);
-}
-
 /// "index()" function
 static void f_index(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 7c4e42420a..4cd81e365d 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -1207,6 +1207,7 @@ static bool viml_func_is_fast(const char *name)
   return false;
 }
 
+/// "vim.call()", aka "vim.fn".
 int nlua_call(lua_State *lstate)
 {
   Error err = ERROR_INIT;
@@ -1224,6 +1225,27 @@ int nlua_call(lua_State *lstate)
     return luaL_error(lstate, "Function called with too many arguments");
   }
 
+  // Fast path: if the vimfn is defined with `func_lua`, call the Lua function
+  // directly without Lua→typval→Object→Lua round-trip.
+  const char *func_lua = find_internal_func_lua(name);
+  if (func_lua) {
+    lua_getglobal(lstate, "require");
+    lua_pushliteral(lstate, "vim._core.vimfn");
+    if (lua_pcall(lstate, 1, 1, 0) != 0) {
+      return lua_error(lstate);
+    }
+    lua_getfield(lstate, -1, func_lua);
+    lua_remove(lstate, -2);  // remove module table
+    // Push args (still on the stack as Lua values).
+    for (int j = 0; j < nargs; j++) {
+      lua_pushvalue(lstate, j + 2);
+    }
+    if (lua_pcall(lstate, nargs, 1, 0) != 0) {
+      return lua_error(lstate);
+    }
+    return 1;
+  }
+
   typval_T vim_args[MAX_FUNC_ARGS + 1];
   int i = 0;  // also used for freeing the variables
   for (; i < nargs; i++) {
diff --git a/src/nvim/types_defs.h b/src/nvim/types_defs.h
index 05c1f12fee..4c00d993ef 100644
--- a/src/nvim/types_defs.h
+++ b/src/nvim/types_defs.h
@@ -32,8 +32,9 @@ typedef double float_T;
 typedef struct MsgpackRpcRequestHandler MsgpackRpcRequestHandler;
 
 typedef union {
-  float_T (*float_func)(float_T);
-  const MsgpackRpcRequestHandler *api_handler;
+  float_T (*func_float)(float_T);
+  const MsgpackRpcRequestHandler *func_api;  // Vimscript bridge to API fn.
+  const char *func_lua;  ///< Lua-implemented vimfn.
   void *null;
 } EvalFuncData;
 
diff --git a/test/functional/core/main_spec.lua b/test/functional/core/main_spec.lua
index 90183efd88..bd1903cc44 100644
--- a/test/functional/core/main_spec.lua
+++ b/test/functional/core/main_spec.lua
@@ -231,6 +231,7 @@ describe('vim._core', function()
       'vim._core.system',
       'vim._core.ui2',
       'vim._core.util',
+      'vim._core.vimfn',
       'vim._init_packages',
       'vim.filetype',
       'vim.fs',
diff --git a/test/functional/lua/vim_spec.lua b/test/functional/lua/vim_spec.lua
index 2fde79a806..00ac46eb88 100644
--- a/test/functional/lua/vim_spec.lua
+++ b/test/functional/lua/vim_spec.lua
@@ -1391,6 +1391,15 @@ describe('lua stdlib', function()
     )
   end)
 
+  it('vim.fn `func_lua` (fast path for Lua-implemented builtins)', function()
+    -- hostname() is implemented via func_lua, calling Lua directly when invoked from Lua.
+    local lua_result = exec_lua([[return vim.fn.hostname()]])
+    eq(type(lua_result), 'string')
+    assert(#lua_result > 0, 'hostname() should return a non-empty string')
+    -- VimScript path (lua_wrapper) should return the same result.
+    eq(lua_result, eval('hostname()'))
+  end)
+
   it('vim.call fails in fast context', function()
     local screen = Screen.new(120, 10)
     exec_lua([[
@@ -2003,14 +2012,14 @@ describe('lua stdlib', function()
 
       local errmsg = api.nvim_get_vvar('errmsg')
       matches(
-        [[
-^vim%.on%_key%(%) callbacks:.*
-With ns%_id %d+: .*: Dumb Error
-stack traceback:
-.*: in function 'error'
-.*: in function 'ErrF2'
-.*: in function 'ErrF1'
-.*]],
+        t.dedent [[
+          ^vim%.on%_key%(%) callbacks:.*
+          With ns%_id %d+: .*: Dumb Error
+          stack traceback:
+          .*: in function 'error'
+          .*: in function 'ErrF2'
+          .*: in function 'ErrF1'
+          .*]],
         errmsg
       )
     end)
@@ -3268,8 +3277,8 @@ describe('vim.keymap', function()
   end)
 end)
 
-describe('Vimscript function exists()', function()
-  it('can check a lua function', function()
+describe('vim.fn.exists()', function()
+  it('can check a Lua function', function()
     eq(
       1,
       exec_lua [[
diff --git a/test/functional/vimscript/eval_spec.lua b/test/functional/vimscript/eval_spec.lua
index 29f17fee19..b4e924104c 100644
--- a/test/functional/vimscript/eval_spec.lua
+++ b/test/functional/vimscript/eval_spec.lua
@@ -215,10 +215,10 @@ describe('listing functions using :function', function()
     command('let A = {-> 1}')
     local num = exec_capture('echo A'):match("function%('<lambda>(%d+)'%)")
     eq(
-      ([[
-   function <lambda>%s(...)
-1  return 1
-   endfunction]]):format(num),
+      t.dedent([[
+           function <lambda>%s(...)
+        1  return 1
+           endfunction]]):format(num),
       exec_capture(('function <lambda>%s'):format(num))
     )
   end)