Are you getting 15fps after the project has been cached for playback? I know I found a bug regarding the new preview caching system. It turns out that if you have a Premiere pro project file in your AE project that it renders and caches a lot slower in 2015.3 compared to 2015.2 2x or 50% slower not sure why this is though
I'd strongly advice you to revert to an older version of After Effects.
The latest one available (13.8.1) has been infested by all sorts of glitches, and the ram preview feels sluggish (specially the audio).
You won't need to re-install most plugins, like the Red Giant suites, because they are installed in a folder which is shared between every version of After Effects and Premiere Pro installed on the computer.