Since the corresponding information is buried in a couple of long threads, here's a quick summary of how to make LibreOffice reasonably fast in Qubes. Use it, if you experience high CPU usage when using LibreOffice and/or LibreOffice to be extremely slow, lag when scrolling, etc.
launch LibreOffice like that:
SAL_USE_VCLPLUGIN=gen libreoffice
Furthermore, in LibreOffice, Tools -> Options -> View * Disable hardware acceleration * Disable anti-aliasing * Enable “Use Skia for all rendering” * Enable "Force Skia software rendering”