Seems now the let clause undertake the overhead:
     Samples    %   Function                                                                                                                                                          
        7230  77% - symbol-overlay-post-command
        7227  77%  - if
        7183  77%   - string=
        7180  77%    - symbol-overlay-get-symbol
        7180  77%     - or
        7176  77%      - thing-at-point
        7163  77%       - save-restriction
        7138  76%        - let
         221   2%         - cond
         206   2%          + let
           2   0%            let*
           7   0%         + if
          40   0%   + symbol-overlay-remove-temp
        1234  13% - command-execute
        1215  13%  - call-interactively
        1205  12%   - funcall-interactively
         783   8%    + next-line
         415   4%    + previous-line
           7   0%    + execute-extended-command
           3   0%   + byte-code
         358   3% + ...
         255   2% + redisplay_internal (C function)
         156   1% + timer-event-handler
          25   0% + undo-auto--add-boundary
          18   0% + jit-lock--antiblink-post-command
          17   0% + internal-timer-start-idle
           8   0% + clear-minibuffer-message

Lars Ingebrigtsen <larsi@gnus.org> 于2021年12月19日周日 22:44写道：
Kang Niu <isgniuk@gmail.com> writes:

> There are no fields in the buffer. (field-beginning) (field-end) return the same value
> as (point-min) (point-max).

Odd.  Does the following fix the performance problem?  I'm wondering
whether it's the field functions that are slow or whether the
narrow-to-region is the thing that somehow triggers the slowness.

diff --git a/lisp/thingatpt.el b/lisp/thingatpt.el
index 2d1bf2013e..e746d94080 100644
--- a/lisp/thingatpt.el
+++ b/lisp/thingatpt.el
@@ -178,7 +178,11 @@ thing-at-point
 See the file `thingatpt.el' for documentation on how to define
 a symbol as a valid THING."
   (save-restriction
-    (narrow-to-region (field-beginning) (field-end))
+    (let ((beg (field-beginning))
+          (end (field-end)))
+      (when (or (not (= beg (point-min)))
+                (not (= end (point-max))))
+        (narrow-to-region (field-beginning) (field-end))))
     (let ((text
            (cond
             ((cl-loop for (pthing . function) in thing-at-point-provider-alist


--
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no