@
jukin -

moving the library to another drive is a good idea.
If that doesn't fix it, try excluding the library from your AV. Many (most) AV's do high frequency silent updates which introduce new 'problems', to justify their existence I guess.
What about calibre's Save to Disk, is it slow?
Another 'trick' would be to install an older version (e.g. 3.28) of 32 bit calibre, it can coexist with the existing 2.33 64 bit install, and it will use the same settings and libraries. That should show whether or not it is a recently introduced problem with calibre -which I very much doubt by the way.
BR