I have noticed that realtime playback during preview is not working unless I am placing the Render Queue Panel under the Timeline Panel. Once that is done playback performance is back to what I would expect, playing the preview smoothly with the proper 24/25 fps.
Of course I have tested this on multiple machines (Macs running Mac OS X El Captian 10.11.3) and this weird workaround fixed playback issues - not sure how or what the results are on Windows or other Mac OS versions though. Hope this helps others as well?
This is a well-documented issue on these forums (more than twenty threads so far). The bug is in El Capitan due to a particular redrawing API. It does not affect Windows.
You can place any panel under there to work around it. Basically, as long as your timeline is up a bit from the bottom (or the preview time indicator isn't visible), you're good.
A much better option: apply to test the beta of 13.7.1 which fixes the issue: help test the After Effects CC 2015 (13.7.1) patch update