I didn't have to be in combat, nor was I playing a long time (had just turned on the PS3)
I have not played with magic much in this game at all, so I'm wondering is this what others are experiencing?
This doesn't seem to be due to long term play.
My toon has only 4 misc quests (like the unavoidable ones for Companions, CoW, Civil War) and only two active proper quests: "before the storm" and "Meridia's"
The striking thing to me is that I have other toons, built on 1.1 and up, that are warrior or assassin (non-magical) types and I never get as bad of FPS as with this one.
I was assigned to fetch her beacon from here, btw http://www.uesp.net/wiki/Skyrim:Ragnvald. Not sure if its strictly due to that dungeon or not? There is a fair amount of water and most enemies seem to be casting spells.

