The huge number of files the server is sending to everyone combined is slowing it down a bit, but once everyone has them things should return to normal.
I've just temporarily slowed down the rate at which the server sends the maps and music which should reduce the in-game lag a bit, but it will probably still be about 24 hours before things calm down to normal levels. After which point I'll speed the download rate up again.
Of course the whole point of this update is to prevent this sort of problem repeatedly occuring everytime we add new content in the future. So if we can just get through this one last big update things should run a lot smoother from then on.