I want to change the speed of an iMovie clip. Where do I start?
You can easily adjust the speed at which a section of your iMovie project (or, for that matter, the entire movie if you want) plays: you can do a John Woo with slow-mo, or add comical sped-up sections for a 1980s Benny Hill aesthetic. This isn't just a temporary speed change: if you export the movie it'll retain the altered-speed sections.
The first job is to select the section of your movie that's going to run at a different speed, and make cuts at each end of the section so that it's a single frame.
Swipe left and right across the movie until the vertical white line (the playhead) sits where you want your fast bit (or slow bit) to start. Tap the clip so that the various action icons appear at the bottom of the screen: Apple calls this the inspector. Then tap Actions, then Split. iMovie will create a cut at the point you selected. Now repeat this process at the end of the section. The designated stretch of footage will separate from the rest of the movie as a discrete frame.
Now make sure the white bar sits within the frame you want to use at a different speed, and again tap to make the action icons appear, and this time tap Speed. Use the slider to adjust the speed: swipe left (towards the slow tortoise icon) to make it slower, or right (towards the hare) to make it quicker.
In either case, the number next to the hare indicates exactly how fast it'll play. The slowest you can go is 1/8th normal speed (well, technically speaking you can go even slower than that, by tapping the Freeze button and choosing how long you want to hold the pose for). The maximum speed is twice normal. The size of the frame will change as you adjust its speed, of course, since it will take up more or less time within the clip as a whole.
As usual when editing, check that the effect is what you were looking for by playing back the entire clip. If you want to adjust the speed again simply get the white playhead marker back in the frame and select another speed, or hit Reset.