Over the past few weeks, I've done a lot of merchandising at the auction house and I've noticed that my in game performance would drop radically after using the interface for a few minutes. The way to reset this apparently was to close the interface and run away to the fountain erasing what I noticed to be a strange greyish mist of smoke that accumulated below me.
Here's a screenshot of the mist in question:
http://i.imgur.com/7EUVs.jpg

Note that it took me a few attempts to get it on screenshot because it disappears almost instantly when I run off and during that time, I have approximately 3fps. When the mist is gone, my fps is improved immediately.
This is consistently reproducible for me.

My rig:
i7 930
6gb ram
gtx 570
windows 7 home premium
Jave and all drivers up to date.

Thanks for looking into this!