I just upgraded to the iPhone 5 and the first thing I did was install my app that I've been working on.
It ran and looked great, and then I minimized it to do something else, but after coming back to it it started running at half speed.
I pressed the home button and then resumed it again, but the problem persisted. Then I did it again and the problem went away.
I can't seem to reproduce it, but there is something wrong with the runtime if it randomly slows down after coming back from being suspended.
I'm going to try and reliably reproduce it next, but I just wanted to report the issue.