First reboot. Second purge all your cache. Then set the Magnification ratio to 50% for a standard HD comp and the resolution to Auto in the preview panel, then try a ram preview. If you are not getting full frame rate previews try skipping 1 frame in the Preview panel. I'm getting full frame rate up to 30 fps on a 2011 Mac Mini with 16 gb ram with these settings for a 1920 X 1080 comp. At 100% I can get real time playback by skipping 1 frame. This will let you preview the audio and video. With 13.6 I'm getting a faster preview by about 20% than I get with CC 2014.
when you say skipping one frame, do you mean you start on frame two, or at 30fps you are skipping every other frame, so previewing at 15fps?
Same here, no more realtime playback as soon as mercury transmit to my blackmagic thunderblot shuttle is turned on.
Look in the preview panel and find the skip frames options. Set that to one and you should see real-time playback.