vim-patch:8.1.0822: peeking and flushing output slows down execution

Problem:    Peeking and flushing output slows down execution.
Solution:   Do not update the mode message when global_busy is set.  Do not
            flush when only peeking for a character. (Ken Takata)
cb574f4154

src/nvim/getchar.c

@@ -2490,8 +2490,10 @@ int inchar(
     }
 
     // Always flush the output characters when getting input characters
-    // from the user.
-    ui_flush();
+    // from the user and not just peeking.
+    if (wait_time == -1L || wait_time > 10L) {
+      ui_flush();
+    }
 
     // Fill up to a third of the buffer, because each character may be
     // tripled below.