Although I primarily use FCPX I finished a project on PP a few weeks ago and was very impressed with how it had improved. Tried starting a project with the latest version and I was initially really impressed with performance improvements however it crashed 3 times in my first session! Hopefully all the crash logs Adobe will be getting will mean there will be a bug fix update shortly.
I’ve had a chance to spend some time with the mobile version and I appreciate that it might be handy for “cloud” project, however as a mobile NLE Lumafusion seems easier to use and is more powerful.
Delete FCPX entirely, go to Purchases in the App store and just reinstall it! If you still get the spinning wheel then it’s an app store problem and you should contact Apple support
First thing – do you have background rendering on in preferences? Do you have the inspector open if you do then does closing it make it any better? If you choose a view without waveforms in the timeline does that help?