I really can’t point to a particular function or two and say “that’s why it needs an OS update.” The program is quite a bit faster than previous versions. I suspect that the some of the underlaying code was rewritten to take advantage of ElCapitan and up – although you’d have to check with someone more knowledgable about such things than I to find the real reason. (Cue Dr. McCoy – “Dammit Jim, I’m an editor-not a programmer.” ☺)
I haven’t had any problems since upgrading from 10.2, although to be honest I really didn’t use it very much before I made the upgrade.
In addition to the COW forums, other good sources of FCPX info are fcp.co ( .co , not .com) and rippletraining.com. If you still run into problems you’re sure to find someone there who knows much more about this than I do.
Mark Landman
PM Productions
Champaign, IL