Among the first things to check are:
- that you’re doing proper RAM previews … with the zero key or SHT+zero keys, not the spacebar … sorry, I use both platforms, but only my fingers (not my brain) recall the Mac keyboard equivalents when I’m on a Win machine as I am now
- that you’re not using a codec that isn’t suitable for AE work … check Dave L.s “stock answer #1” for details
- that you don’t have OpenGL turned on
- that your machine has enough RAM, which a new Mac pro tower should
- that you don’t have a ton of other RAM-intensive or processor-intensive programs and/or background processes running outside of AE
- if you’re working in HD with a massive amount of layers, render-intensive effects, masks, cameras, lights, shadows, etc., etc., you may be getting the best your machine can handle in that scenario
If you;ve confirmed that none of those things are affecting your previews, you may have some other issue going on that’ll require more info to figure out.