There is a (temporary) fix: use “SAL_USE_VCLPLUGIN=gen libreoffice” to start libreoffice (make sure that no other libreoffice-process is already running). Then, libreoffice looks ugly but works without increased cpu usage of xorg.
To add more information: I do have the exact same problem in Debian 11 (Libreoffice 7.1.x) as well as Fedora 34 (Libreoffice 7.2.x): xorg cpu usage goes up to 100% if any action is done within libreoffice in Qubes 4.1.
Special case: open an empty libreoffice-calc document. Select any empty cell: everything runs smooth. Input a simple formula (i.e. “=1+2”) in a cell. Selecting this very cell has a lag and 100% cpu usage in the xorg process do occur.
This behaviour started some weeks ago and was not the case about 2 months ago, even in qubes 4.1. So some update has to be the culprit. Unfortunately, I do not have any good solution other than setting the ugly generic VCLPLUGIN.