I’m having this issue as well. I first noticed the problem yesterday (9/23/12). I had entered Bandithaunt Caverns while on the overflow server for Queensdale. While I was working on a quest in there, I received the notification to switch to the regular server and did so. As soon as I got in my fps went from around 25fps down to literally 4 and has happened every time I’ve entered since. I haven’t had the opportunity to enter the caverns while in the overflow server since, but I have a feeling things will be smooth if I did get that opportunity.