I found that to get UHD footage to play smoothly I have to go down to 1/2 resolution and then after just one effect playback stutters again. In CC 7.2 which I currently use, playback is smooth on full resolution even after two effects.
My issues with CC2015 so far are intermittent dropped frame playback with UHD footage, and audio waveforms not appearing in the timeline until the clip is played or scrubbed. Same footage in CC 7.2 plays perfectly with no audio waveform issue.
OS 10.9.5. I also quit using PPro 2014 and reverted back to CC 7.2.2. Everything works better and all the audio is now synced. 2014 didn’t allow the offset function to work with my Flanders monitor so the program monitor never synced with the Flanders and my HDMI monitor. But the new AJA drivers fixed the dropped frames issue in playback for both versions of PPro. I don’t use multicam much though and I’m afraid of upgrading to Yosemite so far.