I would play around with things to see what helps out but some suggestions would be to play with old models (uncheck everything), turn off spell effects, turn sky off, lower clip plane. Those are some things that I use to lighten the memory (system and video) to reduce lag for myself.
Those are some examples, not to where I can log in to give more options to look at and adjust. I also spoke with a friend that still plays live on a 56K and those are the same things he tweaks (but he says he does it more for his cheaper video card than for his connection).