Virb Edit for Mac - Fast vs Full

I used to be able to edit my Virb videos in full without any jerkiness etc in v.5.2.1, however since that version I have to use Fast (720) in order for it not to be jerky. Problems, is trying to work out what's a good image or not. Why has this changed? Has anyone else had these issues? If I was still using 10.13 I'd simply revert to 5.2.1.. But I'm not. (Or not yet..)