bug#60107: 30.0.50; Open new files very slow if eglot and which-function-mode are enabled.

[Felix]

In GNU Emacs 30.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version
 3.24.35) of 2022-12-15 built on felix-lifebooka531
Repository revision: 0d60579b6b6f2648a881572783322b1bcf931a73
Repository branch: makepkg
Windowing system distributor 'The X.Org Foundation', version 11.0.12201006
System Description: Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-modules --without-libotf --without-m17n-flt --without-gconf
 --enable-autodepend --with-native-compilation=yes --with-xinput2
 --with-x-toolkit=gtk3 --without-xaw3d --without-cairo --with-sound=no
 --with-xwidgets --with-tree-sitter --without-gpm
 --without-compress-install
 '--program-transform-name=s/\([ec]tags\)/\1.emacs/'
 'CFLAGS=-march=native -mtune=generic -O3 -pipe -fno-plt -fexceptions
 -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
 -fstack-clash-protection -fcf-protection'
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
ACL DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON LCMS2
LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11
XDBE XFT XIM XINPUT2 XPM XWIDGETS GTK3 ZLIB

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


Opening C files is very slow if which-function-mode and eglot are
enabled at the same time. I tested this with:

emacs -Q
opening a C project file
M-x eglot
M-x which-function-mode
switching to another .c project file now takes seconds until the buffer
shows up.
I use an old Laptop, but with just one of the two modes disabled,
it's quite snappy.