That's a giant library. Two easy things I'd try: deleting the render cache and deleting FCP prefs.
To delete the render cache, select the library in the browser and choose File->Delete generated library files... Select all render files. Also, to keep FCP from rendering everything, go to FCP prefs and deselect Background render. If you can't play through an effects heavy section, you can manually render just that section.
To delete FCP prefs, launch FCP while holding the option and command keys and select Delete Preferences. FCP will open the default Untitled library. Re-open the library you were working on, set the Prefs back to the way you like (Background render off, Leave files in place, etc.) and see if the issue is resolved.
If you still have problems, post an https://etrecheck.com report here using the Additional text button below in your reply.