bug#73442: 29.2.50; mouse-fixup-help-message is very slow with a slow X connection

[Spencer Baugh via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

I unfortunately don't have a reliable recipe to reproduce this, but:

1. Have a slow X connection, e.g. through ssh -X
2. emacs -Q
3. Move the mouse over links with help messages mentioning mouse-2
4. Experience terrible lag as the mouse moves

I narrowed this down to mouse-fixup-help-message with some profiling,
and (defalias mouse-fixup-help-message #'identity) stops the lag.

I'm not sure why it's so slow.  Maybe posn-at-x-y is particularly slow?


In GNU Emacs 29.2.50 (build 3, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2024-09-23 built on
 igm-qws-u22796a
Repository revision: 340ed90ce4518de238610461047b6c8767ca0cdc
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.10 (Green Obsidian)

Configured using:
 'configure --with-x-toolkit=lucid --without-gpm --without-gconf
 --without-selinux --without-imagemagick --with-modules --with-gif=no
 --with-tree-sitter --with-native-compilation=aot
 PKG_CONFIG_PATH=/usr/local/home/garnish/libtree-sitter/0.22.6-1/lib/pkgconfig/'

Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER X11
XDBE XIM XINPUT2 XPM LUCID ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix