I'm using version 10.1.6 on a 2010 iMac running Hig Sierra. I think this is the point where iMovie has started to suck beyond belief. It used to be good and smooth, but now it is full of inexplicable glitches, slow non-performance and random behaviour. Clips randomly move from one library to the other, projects get mix up or dissappear altogether, only to show back up in months later without any provocation. It seems to have moodswings, where it will import clips of a certain type without hesitation, only to totally refuse to import the exact same type of file the next moment. HD exports sometimes look perfect and sometimes they resemble old VHS-tape. Exporting a 10 minute finished film will sometimes go in 5 minutes and sometimes in 5 hours or even days.
iMovie now behaves like a zoo-monkey on meth: Totally unreliable, erratic and pathetic.