I doubt lag has anything to do with threads with a high number of pages.
I've had lag for 2-page threads (waiting to load the second page), and no lag in threads with tens of pages.

How about we stop trying to speculate for a while, and let Kukri, who can actually do some measurements, let us know in his own good time what he thinks the problem is ?

Speculating, and throwing around ideas, is perfectly good and fine, and can even be productive (see brainstorming), as long as we acknowledge that is exactly that: speculating. We don't know, or have scientifically established, whether threads with multiple pages are slower to load or not, whether forums with more users are slower to load or not, and so on.

If anyone has their own forum online, and can play with it and give us some factual results, great ! Until then, let's not make up solutions for problems that may be inexistant (prune the backroom, make short threads, etc.).