The third expansion map has everything be really, really, laggy. As well as main hubs being really laggy, I'm not sure what to do for the most part, but I do have one big suggestion.
1. Unload Player Characters (PC'S) That aren't attacking anything and aren't in your party. This would massively increase performance in populated areas, and could be a slider where it affects some PC's as opposed to all of them as an option.