[WeirdH]

Okay, this is weird. I just tried a completely fresh install and let the movie database import. Everything was suddenly smooth as ever, but I somehow lost all my 'watched' checkmarks.

Then I proceeded to manually check a few watched movies, and after only a few, all the sluggishness of the GUI was back in full effect! Is this even normal behaviour?

[Karellen]

but I somehow lost all my 'watched' checkmarks.

You need to enable that in advancedsettings.xml as detailed here... https://kodi.wiki/view/Import-export_lib....2F_Import

Your original log is no longer available. Maybe submit a new one which captures you navigating around the menu system and the sluggishness.