I've had this response from ATI after raising a ticket:
Solution: We have seen this issue in our labs and it will be fixed
in a future driver release. In the meantime, please use Catalyst 5.11.
I will ask them which future version of the drivers they expect to include a fix for this problem, but everyone who has this problem should raise a ticket, hopefully it will bump it up in their priority list.
I have tried turning mirrors off. Unfortunately it did not fix the problem
I was getting 130 fps+ with 5.11 drivers, with 5.13 I get 15-20fps!
I have a x800 XL 256MB AGP. I have opened a ticket with ATI, I urge everyone to do so too. I just wish there was a severity rating that I could set to critical