Note that previously, the new forum was dead slow. It is now better, and at least usable, which is a good change (it was the server, I've read).
It is still rather slow on some specialized operations, but I don't consider those important (like, scrolling to down to answer... oh... maybe it is important after all...). It also still requires JS server-side, but I don't think many will be convinced by this argument to keep an old phpbb forum.
I don't know if it's possible, but joining the auth systems would maybe help convincing people (less stuff to do to switch from one to another)?