Yeah, I experienced this issue when I was at the max range of my wireless, when it would drop out some times and therefore be very slow, but I experienced it in 3.1, but I don't know if its a correctable problem, it appears to be an IE problem.
Channel 9 mentions this problem briefly, and says that it might have to do with Javascript and innerHtml, so that would tend to make me think that it has to do with the menus, but not necessarily because of a bug in the menus, but something about slow loading and the menus.