And, to be clear, it is just WebPositive that's lagging. Typing this is unbearable, but if I open, say, Pe (the included text editor) up next to this, it feels just fine. So it's something weird about WebPositive in particular. I may go try Otter or NetSurf (other browsers I can easily install) and see how they feel.
Oddly, though, the YouTube video I had open in the screenshot played fine. It didn't studder or anything like that. The page itself felt awful, but the video worked no problem.
(And, incidentally, no, it wasn't just that I was playing a youtube video. I've closed it and this still feels awful.)