I'm having this problem, too, and also 10.6 unexpectedly crashing [2020 27" iMac, 128G RAM, 3.6 GHz 10-Core Intel Core i9, AMD Radeon Pro 5700 XT 16 GB, running OS 12.0.1]. Spoke with Apple Support twice yesterday — and please know, I love the Apple Support Staff. But in true stereotypical IT fashion, they want to shift the cause of the issue onto the user, i.e., not doing something right, or problems with AVCHD files and 3rd party plug-ins, etc.
Seems to me that if the previous version of FCPX was working just fine with all those conditions, and yet not when updated to 10.6, then that's a problem with the update (or maybe the interface with Monterey, or a combination of the two). I asked that the issue be kicked upstairs to Engineering for a closer look.
This is indeed frustrating, especially when so many of us are on deadlines. One would think that an update would be rigorously tested and spanked before release, even using 3rd party plug-ins and AVCHD like we do in the real world.
Here's one work-around: turning off Background Rendering in FCPX Preferences stops 10.6 from crashing. You'll have to render manually — Render All in the Modify tab. But unfortunately, that doesn't affect the sluggish, snail-like issues.
And of course, we still have the issue which started with the 10.4.9 update where previously rendered material has to be re-rendered every time you launch FCPX. Why does it lose the link with those pre-existing render files? Another colossal waste of time, especially when the clock is ticking and clients are breathing down your neck. If FCPX wants to compete in the Pro market, then these types of issues have to be addressed and fixed ASAP.
For now, I'll re-install the earlier version of FCPX from Time Machine and use that until 10.6 is re-spanked and ready to go.
Cheers and thanks!