Speed button iMovie 10.0.5

Question

The slow motion button does work in this updated version. How do I fix this?

Answer 1

I use a much, much older iMovie so I don't know exactly how the new one works, but until somebody else replies...  It may be re-encoding the whole .mp4 since some formats cannot be edited without doing that, and you would need to check something like format and quality settings to prevent it from saving to a much higher quality which won't actually make the quality any better than the original anyway.