Sounds like a Jerboa issue.
On the site, they’re taking ages to load
Lemmy-ui does that if you've been logged in for a while sometimes. Try clearing your browser cache/data for feddit.uk.
and may not be thumbnails
We don't store thumbnails, what you're seeing are croppings of the full sized images. Most images you'll see here aren't very big though, so this shouldn't be an issue unless your connection is really slow.
Then you can just block the user agent in nginx or whatever you use, like all the other AI scrapers who ignore robots.txt (*cough* Amazon)