The space bar is not for doing Ram Previews. After Effects is NOT a video editor, it is a compositing and motion graphics app specifically designed to create shots or occasionally short sequences. That's all it is. It is very good at that job. Please take the time to go through some tutorials. From your question I see that you were expecting something like an editing app.
Hi Rick, thanks for your response.
I am aware that After Effects is not a video editor, I come from 10+ experience in Adobe products, at work I use iMacs and Macbooks all the time and have never had this problem when it comes to trying to render a simple solid or even nothing. What I'm saying is that now, on my new home iMac, if I try to render NOTHING through the basic preview, I'm getting frame drops, which is something I don't understand.
I'm getting frame drops on a composition that has already been ram previewed and cached, and at work I can press space and get full 25fps playback (of course, if I press space with unrendered footage it will framedrop as it renders it, then if I press space it is back to 25fps playback). I'm getting framedrops on a 4x4 composition with nothing in it, it's like the basic preview is capped at dropping frames no matter what, and it's very frustrating being used to realtime space previewing after the composition has been rendered.
It's possible that the bottleneck is the transferring of frames to your rather large screen using the new operating system. I've seen reports of some of the new aesthetic features in the new Mac OS slowing down a lot of screen drawing operations---not just for After Effects. I'll look back through the posts that I've seen about this and see if I can make a more concrete recommendation.