Just clicking on topics on the forum. Every time you click the entire website has to reload including the menubar at the top and all the advertisements. It takes a long time on Google Chrome. I couldn't imagine the time it would take on Firefox or internet explorer.
Maybe even if you just change the order which loads, so the forum loads first, then the menubar and then the advertisement.
The forums just feel very bulky and are discouraging to use.
I suggest checking out how http://www.cplusplus.com/forum/ works. Their forum loads super fast and is really really basic but is very functional.