Kind of dreading this event - hasn't happened on Oceania yet. I may use the classic client, but don't hate me for it.
Last time there was a single Crimson Dragon my client crashed; first and only time ever. I was as good as dead anyway so it didn't really affect my game.
Another time there were a few shadowlord manifestations, and the client lag was unbelievable.. something like 40 seconds between screen updates. That was the worst.
So yeah, I may cop out and use "Ol' Faithful", at least until they can turn their focus back on the Enhanced Client & get some fixes done.