Q: Can't speed up videos on iMovie.

Waiting on an answer too. This is baffling and may even be a bug if we are lucky. If it is not a feature, then I say iMovie developers are getting complacent. This is 2014, speed up and slow down should standard.

This is also an issue for the company iPad. Why is this speed slider not standardized across the mac platforms? It should be.

Apparently the speed up feature is only available on iPhone 5s, iPad Air, iPad mini with Retina display, and later devices.

It should automatically save the project as you go or at least if you exit into the iMovie library. If you're having problems it may be worth contacting Apple customer support or visiting a store for some more information.