Bumped up to Lion from 10.6.8 and FCP 7 performance was a disaster. I couldn’t navigate along the timeline without “preparing video for display” popping up every time I moved the playhead, and the general response time was exceedingly slow. When I tried to play a sequence, there were drop frames galore, even on clips without filters. I did a fresh install of FCP on top of Lion, and there was no change.
Currently I’m cutting a film shot on the Arri Alexa, using ProRes 4444 files (not proxy) with Nick Shaw’s LUT applied to all, and prior to Lion I could play down unrendered clips without dropping a single frame – so I know it’s not my system.
I wound up backing everything up to an external drive, reformatting my hard drive, reinstalling Snow Leopard and all updates, then reinstalling FCP . A time consuming pain in the ass, but I’ve noticed a small speed boost and most important, Final Cut is humming along.
Bottom line: FCP seems to run flawlessly for some on Lion, while others are experiencing serious issues. I would resist the urge to upgrade, and stay with 10.6.8 if you want to ensure FCP runs as it should.