bug#74337: 29.4; Visiting a file with Unicode emojis is very slow

[Duncan Bayne <duncan@bayne.id.au>]

It takes ~ 1.5 minutes to visit even a small file containing Unicode
emojis, and CPU utilization goes to 100% (of a single core) while it is
loading.

Reproduction steps:

1. Open Emacs with `emacs -Q`.
2. Visit the attached file (known-hosts).
3. Observe that it takes > 1 minute to visit the file.

In GNU Emacs 29.4 (build 1, amd64-portbld-freebsd14.1, GTK+ Version
3.24.43, cairo version 1.17.4)
Windowing system distributor 'The X.Org Foundation', version 11.0.12101013
System Description: 14.1-RELEASE

Configured using:
 'configure --disable-build-details --localstatedir=/var --without-gconf
 --without-libsystemd --without-selinux --with-x --enable-acl
 --with-cairo --with-dbus --with-gif --with-gnutls --with-gsettings
 --with-x-toolkit=gtk3 --with-harfbuzz --with-jpeg --with-json
 --with-file-notification=kqueue --with-lcms2 --without-m17n-flt
 --without-imagemagick --with-mailutils --with-modules
 --without-native-compilation --with-sound=oss --without-libotf
 --without-pgtk --with-png --with-toolkit-scroll-bars --with-sqlite3
 --with-rsvg --with-threads --with-tiff --with-tree-sitter --with-webp
 --without-xft --with-xim --with-xml2 --with-xpm --without-xwidgets
 --x-libraries=/usr/local/lib --x-includes=/usr/local/include
 --prefix=/usr/local --mandir=/usr/local/share/man
 --disable-silent-rules --infodir=/usr/local/share/emacs/info/
 --build=amd64-portbld-freebsd14.1 'CFLAGS=-O2 -pipe
 -fstack-protector-strong -isystem /usr/local/include
 -fno-strict-aliasing ' 'CPPFLAGS=-isystem /usr/local/include' 'LDFLAGS=
 -fstack-protector-strong -L/usr/local/lib ''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBXML2 MODULES NOTIFY KQUEUE PDUMPER PNG RSVG SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2
XPM GTK3 ZLIB

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
cl-loaddefs cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils rmc iso-transl tooltip cconv eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind kqueue lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
emacs)

Memory information:
((conses 16 37686 5905)
 (symbols 48 5447 3)
 (strings 32 13482 2173)
 (string-bytes 1 385060)
 (vectors 16 9591)
 (vector-slots 8 160240 11894)
 (floats 8 21 23)
 (intervals 56 273 0)
 (buffers 984 11))

--
Duncan Bayne
+61 420 817 082 | gemini://duncan.bayne.id.au/

I usually check my mail every 24 - 48 hours.  If there's something
urgent going on, please send me an SMS or call me.