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[Andrew Yoole · Accepted solution]

Unlike a NLE like Premiere Pro, After Effects is a compositing application.  It doesn't play video straight from the file, it loads everything on the timeline into RAM, and then plays the preview from RAM.  As the preview is rendered, a green bar appears at the top of the timeline showing which parts are currently stored in RAM.  In the old days we called that a RAM Preview, but these days it generally just referred to as a preview.

While its rendering the timeline into RAM, it often takes longer than real time to do so, an thats when you see and hear slowdowns.  Once its rendered and the bar is fully green it should play at normal speed. 

After Effects previews should typically be short segments to check your work.  Its very rare that I'd run a preview in AE much longer than 10 seconds.  If you need a long preview, you're better off rendering a temporary output file to view.  

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