Nobody can tell you anything without proper system info or other technical details like comp settings, version of AE, preview resolution and so on.
Adobe After Effects CC 2015, preview settings is on full, only problem I'm having is that it previews only 20seconds instead of the full composition.
Processor 3.2 GHz Intel Core i5
Memory 8 GB 1600 MHz DDR3
Graphics NVIDIA GeForce GT 755M 1024 MB
Software OS X 10.9.5 (13F1112)
If you need any more info I can get it.
The latest AE 2015 update has some issues, I'm sure everyone is waiting for all these to get fixed, my recommendation for you is to uninstall 2015 and install 2014 again. I decided to keep working with 2015 for now, I am too lazy to uninstall and reinstall...It's been about a month and I am seriously considering reinstalling 2014.
It's up to you.
Issues I've been experiencing are:
Keyframe lag - or skipping.
crtl+ z not working properly.
Ram preview not working properly or may be its just me adapting to new way.