This video explains and shows how to do fast and slow motion in iMovie 10.

This tutorial works from iMovie 10.0 to 10.0.x (10.0.3 latest version)

Partial Transcript: So what I can do is take this clip, this clip right here is a very long clip that i want to speed up. So I can shift click it to select all of it…..You can select custom to set a custom percent value or you can click on this dot here on the timeline to change the speed to a slower or faster speed.