I have been having issues with the FPS dropping to slide show levels when a lot of activity is on the screen. Haven't had that issue pre-Dawnguard.
Recenlty wandered near Riften and a group of Bandits was fighting two Cave Bears. When I approched the FPS dropped to 1-2. It was horrendous. I just blocked until some of them killed each other and finally the FPS improved until I could do something.
I have also been having the freezing issue when fast-traveling, or on entering a new area. Don't think I had that once before installing Dawnguard?
I will also add that the vampires killing important NPC is a bit annoying. Maybe tone down the aggro with that?