I just got the Adobe CC suite trial, and when I pull up AE, the preview is to the point where it is so unbearably slow that it might as well just not even be a feature at all. It's not even the fact that it takes anywhere from 5-15 minutes to pre-render a 30 second clip, but it isn't able to preview in realtime on quarter resolution displayed at 15 fps. I have 32 gigs of ram and 28 of those gigs are allocated to AE and still there is no luck. So far I've spent 5 hours unable to even touch the surface of my project because AE isn't able to preview.
I've had projects sometimes that took over a minute to render per frame, so it might not be that unusual to spend 5 minutes to cache 30 seconds worth of project. After Effects is not video editing software and it works an entirely different way under the hood. If you're new to compositing software like this, it can be a bit like culture shock!
Now, as to your realtime preview issues, would I be correct in assuming that you're running El Capitan? If so, press the ` key (grave key) on your keyboard with your mouse in the composition panel to make it full screen and your cached preview should play in real time. Explanation on this bug in Todd's post here: Re: After Effects 2015 won't preview in real time after El Capitan update.