After permissions repair, removing plugins, completely wiping my Mac, a clean install of Mavericks and FCPX I have pretty much concluded that this is a result of upgrading from 10.0.9 to 10.1
However, as you’ll notice from the video I recorded slowdown seems to happen as soon as a project begins to get mildly complex. Hence “pretty much concluded”.
I’ll copy here what i said on the other forum: I would love for someone who is not experiencing these issues to try the title test on a ~30 minute project timeline and confirm that it is not as laggy as I am experiencing. That would confirm that FCPX is capable of complex projects without sluggishness creeping into the UI and I could put it down to something in the upgrade process. Honestly if that is the cause I would quite happy with that outcome.