Without doing any real analysis, i think it's not so much the forum that's slow, but all the crap that now runs down the side. People with older browsers that are limited to the number of GET requests they'll do at a time are going to really suffer imo. The reason chrome/ie9 are much faster is because they do a lot more GETs simultaneously than older browers. IE9 also uses a gpu for rendering. The reality is that a lot of people are still stuck with older browsers at work especially. If the computer is slow as well, it's probably pretty painful.
Using either ie9 or chrome, threads load almost instantly once a link clicks through, and i can start scrolling down, but as I do so, i keep getting pulled back up as individual images load (and these are coming off google).