Hello there, after several tries on deleting FCPX preferences and other suggestions on this specific topic, I decided to make a back up of all my data, including fonts, plugins, transitions fx and QT components. I erased everything on the hard disk and went back OS default settings by reinstalling OS Sierra 10.12.6 and the FXCP I was using. I’m running FCPX 10.3.4 on a MacBook Pro Early 2011, 2,7 Ghz Intel i7, 8 GB memory and it works nice and smooth again! I’m not 100% sure but for me, part of the problem lies on the Quick Time components. Hope this helps.

Hi Sbookings - Thanks for sharing your solution!

Hello, I had the same problems. Until yesterday (13-11-2020) There was an security update and a additional update for MAC OS 10.13.6 (release date 12-11-2020) and the problems are solved!

So that's two instances of this issue where the problem apparently was OS-related. I guess that's not surprising -- as Iain pointed out FCPX is moving away from supporting those older OS versions (the latest version apparently requires Catalina).