Post-command hooks are slow to run for commands invoked using `M-x ...`.

I'm using Emacs 26.3 (which produced this bug report), but I've checked
with 27.2.2 and I get the same behaviour.


Here's a startup file that can be used to show this (also at
https://github.com/simon-katz/demo-post-command-hook-slow-with-m-x-commands.el/blob/8e7b96fc322f6372fb1531f04ab6008d59ab9b9e/demo-post-command-hook-slow-with-m-x-commands.el#L1
).

;;; demo-post-command-hook-slow-with-m-x-commands.el --- Demo that
post-sommand hooks are slow with M-x commands      -*- lexical-binding: t;
-*-

;; This demonstrates an issue with `(add-hook 'post-command-hook ...)`.

;; To reproduce the problem:
;; - Run `emacs -q -l demo-post-command-hook-slow-with-m-x-commands.el`
(that's
;;   this file).
;; - Enter `M-x next-line`.
;; - Observe that the message displayed by the `say-something` function
;;   takes a while to appear.
;; - Contrast with hitting the down-arrow key, when the message appears
;;   immediately.

;;; Code:

(require 'package)

(setq debug-on-error t
      no-byte-compile t
      package-archives '(("melpa" . "https://melpa.org/packages/")
                         ("gnu" . "https://elpa.gnu.org/packages/"))
      package-user-dir (expand-file-name (make-temp-name
"flycheck-tmp-elpa")
                                         user-emacs-directory)
      custom-file (expand-file-name "custom.el" package-user-dir))

(defun say-something ()
  (let ((inhibit-message t))
    (message
     "With `M-x next-line` (for example), this message is slow to appear")))

(add-hook 'post-command-hook 'say-something)

(display-buffer "*Messages*")

(provide 'demo-post-command-hook-slow-with-m-x-commands)
;;; demo-post-command-hook-slow-with-m-x-commands.el ends here






In GNU Emacs 26.3 (build 1, x86_64-apple-darwin18.2.0, NS appkit-1671.20
Version 10.14.3 (Build 18D109))
 of 2019-09-02 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.1894
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
With ‘M-x next-line‘ (for example), this message is slow to appear [13
times]
You can run the command ‘next-line’ with C-n
With ‘M-x next-line‘ (for example), this message is slow to appear [10
times]
Quit
With ‘M-x next-line‘ (for example), this message is slow to appear [7 times]
Making completion list...
With ‘M-x next-line‘ (for example), this message is slow to appear [6 times]

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
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