For some, I saw that they had performance areas in even parts of the main game. I haven't experienced that, but I also haven't hit all the old areas yet. I've been going back and forth, and in particular to the new areas. Framerates start out at 44/45, and then bottom out to 22-25 during combat. It's horrible.
During one of the last beta updates, I loaded a saved game in which I was in one of the new white march areas, and it bugged out to where none of my characters could be seen. The first thing I noticed, however, was that my framerate was back up to 90. When I reloaded a different saved game in which I could see my characters it dropped back to 45. So it must have something to do with how the characters are rendered?