Looks like there are three separate issues. A menu (possibly general) slowdown problem for users of multiple monitors, such as myself, that is fixed by playing with only one monitor enabled.
A crashing bug when transitioning areas with no known cause or workaround yet, which I am not personally experiencing, though I have two machines to test it on and I've only played on one so far.
Also I think there are some small memory leaks, if I watch the game in task manager it will slowly use more and more memory, though it's not causing any issues for me.
Since there's been one guy who said he used three monitors with Eyefinity without the slowdown I'd hazard the guess that the game engine just wasn't built to accommodate multiple monitors and is doing something very silly with overlays.
I did a little experimenting, and it appears that duplicating the display on two monitors, as opposed to extending the desktop, doesn't cause the menu problems. I haven't done a lot of testing, since I don't have a lot of time today, but maybe someone else can try this and post their results. Also it appeared from a very short test, too short to be conclusive, that I could start the game with only one active display, then switch to extended desktop mode - the mode that I suspect is the reason for everyone's problems - and not have the problems. If more people can confirm this then it's probably a problem during initialization.