Sometimes computers get laggy after a while, either due to having too many open windows, or just for the fact that, after a while, Windows just needs to be shut down and rebooted, in order to iron out any kinks, and refresh the RAM and swap file. Perhaps a simple fix of shutting down and rebooting may fix the issue. If not, there could be something else going on, like a software conflict with something else other than LibreOffice, or perhaps even a faulty app installation. You may need to do a purge and reinstall after redownloading the software.
