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I was almost convinced to reinstalling iMovie 3, but the last 2 posts stating that it's usuable in a 500 Mhz iBook but playback of clips is slow is exactly what's wrong with my previous install. Once loaded I can move around respectably, but when I play a project (4 min video with transitions, stills, clips, and an mp3 as background track) and its terribly slow. I look at my time-index and it moves at 2 sec increments. Aweful!. iMovie 2 playback was smooth, therefore I can preview my project prior to exporting.

Cheers,
Tex
 
check full screen playback

On one of my emacs, I have the same problem, and originally thought another process must be interupting iMovie, but the problem disappears when playing back full screen.
 
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