I've been having slowdowns on nVidia as well, it has something to do with the number of player characters on screen, I haven't monitored it yet, but I think my GPU runs out of memory and has to start swapping. It runs fine@30fps+ usually, but for example yesterday in Drybone it was pretty bad with drops to <10fps. I don't think it's a driver problem, but a performance problem. They should add an option to limit visible models dynamically depending on your GPU.