diff --git a/runtime/doc/fold.txt b/runtime/doc/fold.txt
index 8f7393f5e3..b844e0ed85 100644
--- a/runtime/doc/fold.txt
+++ b/runtime/doc/fold.txt
@@ -69,8 +69,6 @@ method.  The value of the 'foldexpr' option is evaluated to get the foldlevel
 of a line.  Examples:
 This will create a fold for all consecutive lines that start with a tab: >
 	:set foldexpr=getline(v:lnum)[0]==\"\\t\"
-This will call a function to compute the fold level: >
-	:set foldexpr=MyFoldLevel(v:lnum)
 This will make a fold out of paragraphs separated by blank lines: >
 	:set foldexpr=getline(v:lnum)=~'^\\s*$'&&getline(v:lnum+1)=~'\\S'?'<1':1
 This does the same: >
@@ -79,6 +77,10 @@ This does the same: >
 Note that backslashes must be used to escape characters that ":set" handles
 differently (space, backslash, double quote, etc., see |option-backslash|).
 
+The most efficient is to call a function without arguments: >
+	:set foldexpr=MyFoldLevel()
+The function must use v:lnum.  See |expr-option-function|.
+
 These are the conditions with which the expression is evaluated:
 - The current buffer and window are set for the line.
 - The variable "v:lnum" is set to the line number.
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 875767283a..a48d020a34 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -400,6 +400,14 @@ Set using a variable with lambda expression: >
 	let L = {a, b, c -> MyTagFunc(a, b , c)}
 	let &tagfunc = L
 
+Calling a function in an expr option			*expr-option-function*
+
+The value of a few options, such as 'foldexpr', is an expression that is
+evaluated to get a value.  The evaluation can have quite a bit of overhead.
+One way to minimize the overhead, and also to keep the option value very
+simple, is to define a function and set the option to call it without
+arguments.
+
 
 Setting the filetype
 
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index b8c3df5688..c8451b7a28 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -1349,7 +1349,7 @@ int eval_foldexpr(win_T *wp, int *cp)
   const sctx_T saved_sctx = current_sctx;
   const bool use_sandbox = was_set_insecurely(wp, kOptFoldexpr, OPT_LOCAL);
 
-  char *arg = wp->w_p_fde;
+  char *arg = skipwhite(wp->w_p_fde);
   current_sctx = wp->w_p_script_ctx[WV_FDE].script_ctx;
 
   emsg_off++;
@@ -1360,8 +1360,23 @@ int eval_foldexpr(win_T *wp, int *cp)
   *cp = NUL;
 
   typval_T tv;
+  int r = NOTDONE;
+
+  // If the expression is "FuncName()" then we can skip a lot of overhead.
+  char *parens = strstr(arg, "()");
+  if (parens != NULL && *skipwhite(parens + 2) == NUL) {
+    char *p = strncmp(arg, "<SNR>", 5) == 0 ? skipdigits(arg + 5) : arg;
+    if (to_name_end(p, true) == parens) {
+      r = call_simple_func(arg, (int)(parens - arg), &tv);
+    }
+  }
+
+  if (r == NOTDONE) {
+    r = eval0(arg, &tv, NULL, &EVALARG_EVALUATE);
+  }
+
   varnumber_T retval;
-  if (eval0(arg, &tv, NULL, &EVALARG_EVALUATE) == FAIL) {
+  if (r == FAIL) {
     retval = 0;
   } else {
     // If the result is a number, just return the number.
@@ -1428,6 +1443,31 @@ Object eval_foldtext(win_T *wp)
   return retval;
 }
 
+/// Find the end of a variable or function name.  Unlike find_name_end() this
+/// does not recognize magic braces.
+/// When "use_namespace" is true recognize "b:", "s:", etc.
+/// Return a pointer to just after the name.  Equal to "arg" if there is no
+/// valid name.
+static char *to_name_end(char *arg, bool use_namespace)
+{
+  // Quick check for valid starting character.
+  if (!eval_isnamec1(*arg)) {
+    return arg;
+  }
+
+  char *p;
+  for (p = arg + 1; *p != NUL && eval_isnamec(*p); MB_PTR_ADV(p)) {
+    // Include a namespace such as "s:var" and "v:var".  But "n:" is not
+    // and can be used in slice "[n:]".
+    if (*p == ':' && (p != arg + 1
+                      || !use_namespace
+                      || vim_strchr("bgstvw", *arg) == NULL)) {
+      break;
+    }
+  }
+  return p;
+}
+
 /// Get an Dict lval variable that can be assigned a value to: "name",
 /// "name[expr]", "name[expr][expr]", "name.key", "name.key[expr]" etc.
 /// "name" points to the start of the name.
diff --git a/src/nvim/eval/userfunc.c b/src/nvim/eval/userfunc.c
index ab8e67016f..f7d1e7e0f8 100644
--- a/src/nvim/eval/userfunc.c
+++ b/src/nvim/eval/userfunc.c
@@ -1561,12 +1561,12 @@ varnumber_T callback_call_retnr(Callback *callback, int argcount, typval_T *argv
 
 /// Give an error message for the result of a function.
 /// Nothing if "error" is FCERR_NONE.
-static void user_func_error(int error, const char *name, funcexe_T *funcexe)
+static void user_func_error(int error, const char *name, bool found_var)
   FUNC_ATTR_NONNULL_ARG(2)
 {
   switch (error) {
   case FCERR_UNKNOWN:
-    if (funcexe->fe_found_var) {
+    if (found_var) {
       semsg(_(e_not_callable_type_str), name);
     } else {
       emsg_funcname(e_unknown_function_str, name);
@@ -1686,12 +1686,9 @@ int call_func(const char *funcname, int len, typval_T *rettv, int argcount_in, t
   }
 
   if (error == FCERR_NONE && funcexe->fe_evaluate) {
-    char *rfname = fname;
-
-    // Ignore "g:" before a function name.
-    if (fp == NULL && fname[0] == 'g' && fname[1] == ':') {
-      rfname = fname + 2;
-    }
+    // Skip "g:" before a function name.
+    bool is_global = fp == NULL && fname[0] == 'g' && fname[1] == ':';
+    char *rfname = is_global ? fname + 2 : fname;
 
     rettv->v_type = VAR_NUMBER;         // default rettv is number zero
     rettv->vval.v_number = 0;
@@ -1765,7 +1762,7 @@ theend:
   // Report an error unless the argument evaluation or function call has been
   // cancelled due to an aborting error, an interrupt, or an exception.
   if (!aborting()) {
-    user_func_error(error, (name != NULL) ? name : funcname, funcexe);
+    user_func_error(error, (name != NULL) ? name : funcname, funcexe->fe_found_var);
   }
 
   // clear the copies made from the partial
@@ -1779,6 +1776,58 @@ theend:
   return ret;
 }
 
+/// Call a function without arguments, partial or dict.
+/// This is like call_func() when the call is only "FuncName()".
+/// To be used by "expr" options.
+/// Returns NOTDONE when the function could not be found.
+///
+/// @param funcname  name of the function
+/// @param len       length of "name" or -1 to use strlen()
+/// @param rettv     return value goes here
+int call_simple_func(const char *funcname, int len, typval_T *rettv)
+  FUNC_ATTR_NONNULL_ALL
+{
+  int ret = FAIL;
+
+  rettv->v_type = VAR_NUMBER;  // default rettv is number zero
+  rettv->vval.v_number = 0;
+
+  // Make a copy of the name, an option can be changed in the function.
+  char *name = xstrnsave(funcname, (size_t)len);
+
+  int error = FCERR_NONE;
+  char *tofree = NULL;
+  char fname_buf[FLEN_FIXED + 1];
+  char *fname = fname_trans_sid(name, fname_buf, &tofree, &error);
+
+  // Skip "g:" before a function name.
+  bool is_global = fname[0] == 'g' && fname[1] == ':';
+  char *rfname = is_global ? fname + 2 : fname;
+
+  ufunc_T *fp = find_func(rfname);
+  if (fp == NULL) {
+    ret = NOTDONE;
+  } else if (fp != NULL && (fp->uf_flags & FC_DELETED)) {
+    error = FCERR_DELETED;
+  } else if (fp != NULL) {
+    typval_T argvars[1];
+    argvars[0].v_type = VAR_UNKNOWN;
+    funcexe_T funcexe = FUNCEXE_INIT;
+    funcexe.fe_evaluate = true;
+
+    error = call_user_func_check(fp, 0, argvars, rettv, &funcexe, NULL);
+    if (error == FCERR_NONE) {
+      ret = OK;
+    }
+  }
+
+  user_func_error(error, name, false);
+  xfree(tofree);
+  xfree(name);
+
+  return ret;
+}
+
 char *printable_func_name(ufunc_T *fp)
 {
   return fp->uf_name_exp != NULL ? fp->uf_name_exp : fp->uf_name;
@@ -3248,7 +3297,7 @@ static int ex_defer_inner(char *name, char **arg, const partial_T *const partial
       if (ufunc != NULL) {
         int error = check_user_func_argcount(ufunc, argcount);
         if (error != FCERR_UNKNOWN) {
-          user_func_error(error, name, NULL);
+          user_func_error(error, name, false);
           r = FAIL;
         }
       }
diff --git a/test/old/testdir/test_fold.vim b/test/old/testdir/test_fold.vim
index 36f72f5e01..a0eb3afdbb 100644
--- a/test/old/testdir/test_fold.vim
+++ b/test/old/testdir/test_fold.vim
@@ -386,6 +386,32 @@ func Test_foldexpr_no_interrupt_addsub()
   set foldmethod& foldexpr&
 endfunc
 
+" Fold function defined in another script
+func Test_foldexpr_compiled()
+  throw 'Skipped: Vim9 script is N/A'
+  new
+  let lines =<< trim END
+      vim9script
+      def FoldFunc(): number
+        return v:lnum
+      enddef
+
+      set foldmethod=expr
+      set foldexpr=s:FoldFunc()
+  END
+  call writefile(lines, 'XfoldExpr', 'D')
+  source XfoldExpr
+
+  call setline(1, ['one', 'two', 'three'])
+  redraw
+  call assert_equal(1, foldlevel(1))
+  call assert_equal(2, foldlevel(2))
+  call assert_equal(3, foldlevel(3))
+
+  bwipe!
+  set foldmethod& foldexpr&
+endfunc
+
 func Check_foldlevels(expected)
   call assert_equal(a:expected, map(range(1, line('$')), 'foldlevel(v:val)'))
 endfunc