First, if you have an Intel Mac, After Effect will be slower than it should be (it’s not “universal”).
Second, you have different choices for your preview: Wireframe, Adaptive resolution, Open GL-interactive, Open GL always on. To chose one of these click on the icon at the bottom of your preview window (composition window). To use Open GL, you need a video card with Open GL. To check if you have one of these card, go into you preferences under “preview” option and click on Open GL info. You can also set your adaptive resolution limit.
And last, the more you add effects, layers, etc, the more your computer and after effects will have difficulties to be “real time scrubbing”
I hope it can help.
May the force be with you