They run special builds during the demos that presumably only have a portion of the functionality to ensure that no bugs pop up. You'll notice that they stick very closely to a script that is on the desk while they demo - sometimes you can see them looking at it and flipping the pages.
Not only that, but it's been confirmed that the iOS Beta version they ran was newer than what was released to the developers.Three words: "Screen Images Simulated"
It's probably 3 months newer code. The beta file creation date is march. I'm sure the demo code was only a few days old in comparison...
you running a 5? I just went from a 4 to a 5 and ios7 runs super smooth imo. especially considering its a beta