Thanks wseiber, paulb and belM. I hadn't twigged that it related to choosing a particular page rather than Show All, and my testing of that theory has been pretty inconclusive since the behaviour doesn't appear to be consistent. I always used to select Show All, but recently I've been hitting just the last page if I know that it's a thread I'm pretty much up to date with, in the vain hope that it will load more quickly that way.

After I posted the question here, I decided to mail it to Anu directly. If he comes up with any other helpful suggestions, I'll post them here for all.

I've also noticed that "word wrap" sometimes doesn't happen on posts, i.e. there are some posts, or perhaps it's only on some login sessions, that display with very long text line lengths (several "screens" wide) so that you have to keep scrolling out to the right to read them. Others stay nicely inside the window. Can anyone explain that phenomenon?