I installed iMovie on my macOS 10.14.2 MacBook Pro, created an iMovie project, imported your movie into it, saved it out as a new movie, quit iMovie, and then launched PM5 and browsed a folder full of various still image formats that PM5 supports. Speed was fine, previews loaded quickly.
If you're not running 10.14.2 (I saw on one of your earlier posts that you were using 10.14.1) then please do that upgrade.
Beyond that, I'm not sure what can be done to solve your problem. Perhaps there is something wrong with your new system.
-Kirk