For my own reference, were there any other spots in the game which had a framerate dive like that? or was it just Act IV? (from your initial post it seems like the rest of the game is performing pretty decently)
Only Act IV, the rest of the game handled really well. There's maybe a small drop when the Missirt starts doing flips or getting stuck in geometry, but it's hard to tell with said flips and physics going in all directions.
