I have LibreOffice 6.1.5.2 and it consumes a lot of processing power (over 90%) in the background when opened and running but not used by me for the moment. As soon as I shift focus to a LibreOffice window the processor activity drops to 10% or less.
This reads like a bug. But note that LO 6.1 is already old and unsupported. LO still is 6.3 and LO fresh 6.4, so I suggest to try an updated LO version for your operating system. Or you can try a parallel install of a newer version, as described in Installing several versions of LibreOffice in parallel - The Document Foundation Wiki.
Thanks for the comment. I will try updating LibreOffice.