I have the same issue too. Editing a 1080p video on Final Cut Pro and every 20 to 30 minutes, the spinning beach ball appeared. For now, the temporary solution is to quit FCPX and open it again. Apple needs to fix this issue with an update of FCPX or Catalina ASAP. I bought the new MacBook Pro 13inch (2020) just to have a more powerful machine to edit my videos, but it seems like editing on my 2015 MacBook Air is much smoother.
Here's my Mac details:
macOS Catalina
Ver 10.15.5
MacBook Pro (13-inch, 2020, Four Thunderbolt 3 ports)
2 GHz Quad-Core Intel Core i5
16 GB 3733 MHz LPDDR4
Intel Iris Plus Graphics 1536 MB
FINAL CUT PRO X:
Ver 10.4.8