I've noticed that playback performance is not too solid with Final Cut Pro X and Decklink (both PCI and Thunderbolt based). When I play back at anything other than 1x the Decklink output to the broadcast monitor goes out of sync with what's playing back on the computer monitor. It turns into a slideshow at higher speeds. If I'm shutting footage backwards or forwards and then stop playback, the broadcast monitor "catches up" with where playback actually is, so looking at the Decklink output while editing is frustrating- I keep my eyes on the Mac display to see where I really am.
Is this a Decklink problem or an inherent limitation of AV monitoring in FCPX? We are a shop with 10+ Decklink cards so I don't have another solution to compare, but I would consider switching to AJA if it gets the job done.
I have noticed this in every version of the Decklink driver since I switched over to X in December 2012- I keep the latest OS and Blackmagic drivers installed at all times.
FWIW, the same Decklink on the same machine has better results in FCP7, except for the random audio crackling that many here are experiencing.
I think this primarily is a FCPX issue. I've got a AJA Kona 3 in the main suite (MacPro) and I've experimented with a Blackmagic Ultra Studio Monitor (2011 MBP, 2012 iMac) and experience similar issues with A/V lag out to the reference monitor. Some people claim "the new iMac make wonders in FCPX!" but they don't fix this issue from my experience.
I also talked about this issue with AJA at IBC last year and a few weeks back and both times the answer has been "yes, we're aware of the issue and we're talking to Apple about it". A pretty crappy for a so called "pro" solution if you're in need of reference monitor monitoring.