Which browser are you using?
I've been using Chrome, and I've noticed having trouble with posts/threads that feature a large number of GIFs - running into them causes it to retain a whole bunch of them in memory. Finding the relevant process in the task manager and removing it solves the lag problem, but it also might crash the page, so you should make sure to finish up any post before trying it. If you're using, say, Firefox, it's a bit more complicated, but closing all your open Pokébeach tabs before opening any new ones should have a similar effect.
Pretty sure the problem you're describing is the GIFs thing, though, since most of this forum's user's wouldn't encounter it, and therefore wouldn't be affected in the same way.