They probably did a quick & dirty just to get it working with 1.04, hopefully there will be an update soon. I'm on a 4ghz i7 with SLI 670 GTX and even with all that it will drop down to like 2 or 3 FPS sometimes.
I'm not getting the FPS issues that hard but there does seem to be a bit of a performance drop here and there. Normally smooths back out. My D3 did just crash mid-leap (barb act 1 champ farming) but I'm not sure what end that problem is on. Also using Belphegor.
Getting the same occational drop in fact I always get it, even before the new update. Running an I7 with 16gig RAM, 500GB SSD. Occationally D3 freezes and stutters. I've attached my latest log with diagnostic logging.