Don't know if anyone has some thoughts they can share.
I'm a long way through a really intensive project with FCPX and it has handled it really well, with some of the updates bringing serious speed improvements.
I've run into a brick wall with something though. Media is 40min r3d 4K cam files x2 with a 1080p BMCC side camera and an external audio track. Since the last update FCPX has been syncing these multicam sequences without any issues in less than a minute. Now I'm finding myself waiting for nearly 5mins for them to work and usually giving up halfway through and doing it myself.
work is stored on a G-Tech 48TB RAID via thunderbolt 2, working on a new mac pro 6 core, 64GB RAM, D700s.
Definitely strange, although i went through a similar issue not so long ago and I suspect something had corrupted with the FCPX install. I did a clean install the other day and multicam syncing is now the fastest I can remember it being in months.
One thing that can help when it's going slow is to add a marker at roughly the same point (it doesn't have to be exact) in each clip..
On the plus side, as someone once told me during a Sony Vegas masterclass back in the day, that five or ten minute wait is the perfect opportunity to grab a coffee, find some sunshine, relax, and take what is probably well-earned break.