Framedrop in Piety's arena

Hi,

I was just playing through lunaris temple with a resolution of 1650 x 1050 like i always do. The FPS is fine on about 60 FPS. Then i went through the portal to Piety's arena, but the frame dropped till about 5 FPS (i checked with F1). It was completely unplayable so i went back to town. The FPS was back at 60 again and everything was fine again.
So i tried again to Piety's arena but still the huge framedrop. Last week or 2 weeks ago i didnt had any problem in Piety's arena.

I actually also get this frame drop. Not as severe as yours, and I'm still able to kill her, but it more than halves my FPS there.