Playhead & Spacebar

When you preview in After Effects, what's actually happening is that AE is rendering each frame into your computer's RAM.  AE is very good at doing this these days so it can often appear to be happening instantly.  But the more effects or layers you add, the longer AE will take to process the animation.  Once your RAM is full or the entire timeline is rendered into RAM, AE will playback smoothly.

Some useful tips are:

• Drop the quality from Full to Auto - this helps you check timings, but the image may not be as sharp
• Solo just the layers you want to check - again helps with timing.
• Build up the preview first, by hitting the '0' key on the numberpad - Have your phone or tablet at the ready to provide distraction for a couple of minutes.