Managed to fix this in a fairly agonising fashion:
- Open the Asset Library
- Every time a screen of thumbnails fully loads, scroll down to see more 'loading' thumbnails.
- Keep doing this scrolling bit by bit until all the thumbnails are loaded. 15-20mins of active, very dull use.
- It doesn't seem to load if you just leave the program by itself. You have to keep scrolling down to the next blank thumbnails or it stops loading.
- If you scroll up after getting to some 'loading thumbnails', everything previously loaded will disappear. These have not actually un-loaded though, if you click a Tag and unclick a Tag they'll come back (this is not necessary but at least reduces your rage level).
- Once every thumbnail is loaded, you can scroll up and down without the whole thing breaking.
You can at least use another program while it's loading each page of thumbnails, but you do have to keep going back to scroll down or it stops loading the next bunch, at least for me.
Once it's fully done the Library is still clunky (3 second delay to turn a tag on, 10 second delay to turn a tag off) but at least it's functional, and you only have to do this once.
To see the problem, Enscape devs should just need to do a fresh install and use an internet connection to load the assets. It happens on every install here.