My libraries (that used to take seconds to load) now take between 15 and 30 minutes to load.
In a streamed console window (as per the suggestion in the replied to post), I got 230,000 errors/faults just while opening Final Cut and the library! No wonder it's slow.
I'm editing with a client next week and it's going to be embarrassing to have to say, hold on while my otherwise fast MacBook Pro 16" takes half an hour to load your project. I hope Apple updates either Final Cut or Monterey or both asap.
I've tried the usual stuff like resetting FC settings, reinstalling FC and so on. I even rolled back to 10.5.4 on Monterey and had the same issues. I've run SSD First Aid, rebooted in safe mode, removed CurrentVersion.flexolibrary to rebuild the library database (or whatever it does), removed individual events manually from the bundle and replaced, removed render files manually, removed all plugins from the Movies - Motion Templates folder, run an Etre Check report (nothing major), and nothing helps.
Once the library or event loads, editing is more or less normal, although it is a bit slower than before.
The only thing that didn't have a problem was an older MBP 13 with Big Sur 11.5.2 on it. This was absolutely fine with the same libraries using both FC 10.6 and 10.5.4.
I think this is a Monterey problem. Or at least a Final Cut with Monterey problem, and not necessary a Final Cut v 10.6.