I’ve finally discovered what the problem is. It is associated with GTK3 rather than Libreoffice, the linux kernel, or the graphics drivers.
Changing the theme from GTK3 to QT5, then logging out and back in fixes the problem. Saving is once again instantaneous the way it used to be under GTK2.
/etc/profile.d/libreoffice-fresh.sh
export SAL_USE_VCLPLUGIN=qt5
/etc/profile.d/libreoffice-fresh.csh
setenv SAL_USE_VCLPLUGIN qt5
This also explains why multiple versions of Libreoffice are affected. The last usable version without changing to QT5 for me was 6.1.6.3
It might also explain why there are so many weird bugs in xfce, particularly associated with thunar and the clipboard plugin.