There have been quite a number of issues with the new version of AE. known issues in After Effects CC 2015 (13.5) | After Effects region of interest
Now, as you'll note from that link, a number of those were fixed in the 13.5.1 update. Also, a number of errors in AE were fixed when Apple squashed bugs in Yosemite on their end. So, make sure your AE is updated to 13.5.1 and make sure, if you have Yosemite, that you've updated it to 10.10.4
The reason for all of these issues is that After Effect CC 2015 is the first step in a major overhaul of AE's architecture. This is a bigger deal than when it went from 32-bit to 64-bit in CS5! Good news for the future perhaps, but a bit of a rocky road right now! The preview system, for example, is very different: all about previews in After Effects CC 2015 (13.5) | After Effects region of interest
According to the Adobe folks, the intent of splitting the UI from the rendering thread on the processor is meant to have somewhat of the opposite effect to what you're describing. And, for me things are running smoother with CC 2015.
I'm curious of your issue is just that you're working on really complex or large frames and, as such, it requires a lot of rendering power to show a change. Multiprocessing isn't present in CC 2015 (see this page features not available in After Effects CC 2015 (13.5) | After Effects region of interest), because replacing multiprocessing is the major goal of the AE architecture change.
Is this sort of interface slowdown you're describing consistent across various types of compositions with or without footage? What sort of compositions are you working in? Are we talking 4K or 1080p or something else? What sort of footage are you applying these effects to when you see this kind of issue (frame size, format, codec, etc.)? I'm