I ran into this problem today as well. My FPS dropped into the 5 - 15 range while playing on my 110 Shaman. Everything went back to normal once I disabled the Carbonite Quests addon.
I've also noticed my minimap is acting a bit differently as well. For some reason, some of my icons are huge, and some of them look as though they're cut in half. Here's a screenshot of what I'm seeing.
