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.