TL;DR: I have exactly the same issues on my 2020 13" MBP.
I just wanted to chime in that I have the same issue. 2020 MacBook Pro 4-TB3 ports, 2.3GHz i7, 32GB, Iris Plus and 4TB disk. FCPX works like a dream for the first 15-30 minutes, then I get one beach ball, and it goes away, and then they keep coming and it gets completely unbearable. within minutes. I managed to reboot myself through editing a 50min video earlier this month, but now I'm back working on another one and the same issues. I've updated all the software I can find, and all of that.
Hoping to get a resolution to this soon - it's pretty ridiculous.