while in Mist drop your draw distance to minimum.

The game may try and render stuff miles off but the mist prevents you seeing it so why not reduce the overhead on your card that way at least?

The mist itself seems to be the source of a lot of it. When I've modded the mist away with the MistBeGone mod, the FPS dramatically improved. Don't know if that's a solution you want to use, though.