I have the same issue since 2015. I cant work my way around it and I cant find a proper fix.
I am working in a iMac Late 2013 with NVIDIA GeForce GTX 780M.
The preview just doest work, and never plays back in proper FPS.
Definitely frustrating, was working great until the 13.6 update. I'm running a hp z840 win 7 pro w/Nvidia quadro k5000.
If you'd like to help us to test the After Effects CC 2015 (13.6.1) update---which fixes this bug---you can request access to the prerelease testing program here:
Choose the "Private programs" tab and click Apply to apply for the "After Effects CC 2015 (13.6.1) patch update" program.
The After Effects CC 2015 (13.6.1) bug-fix update is available. It fixes several bugs, including the one being discussed on this thread.
Let us know how things work after you have installed the update.