got the same issue with the new mac pros at work. apparently there are no fixes for it yet. I´ve been previewing in the slow frame rate and rendering in adobe encoder whenever I need to check more precisely my animation. a real pain.
one thing you can try is to update your AE and also import setting from an older version. ( go to preferences> general, one of the last options there)
Use AE 2014. AE 2015 is a bug ridden proof of concept (alpha level build) and the renderer is a known weak point with a raft of issues. Unfortunately Adobe don't give you a warning when you're installing it, nor when you're rendering and previewing, as they should. It's really that bad.
At times 50% of this forum is dedicated to this problem. Never less than 25% of the top posts are about the issues with AE 2015.
In other news, you'll find 2014 runs faster on your machines in Windows 7 run in Bootcamp.
There's also a slight lag before previews run stably in AE 2014, but it's nothing like the issues with 2015.
Or you could try BlackMagicFusion 8's beta. It's free, but has the huge caveat: You need to learn a purely nodal interface to get the most out of it. Which can be (is) weird.