I seem to have the same issue. And it seems to vary depending on the GTK_THEME.
When setting the environment variable GTK_THEME=Adwaita it seems less worse. Without the variable my GTK_THEME seems to be Clearlooks-Phenix. (I kept the clearlooks-phenix-theme DEB from Debian-10 installed on Debian-12. I know, that's probably not recommended and not supported.)
I think these could maybe be related: TBird occasionally gets VERY slow for several seconds or more... https://bugzilla.mozilla.org/show_bug.cgi?id=1935803 Extremely slow to load messages and getting notifications https://bugzilla.mozilla.org/show_bug.cgi?id=1935903 Hinweis: accessibility.force_disabled=1 setzen Regards, kolAflash
