http://www.1up.com/do/previewPage?pager.offset=1&cId=3151120
I guessed right for a change...
1UP: The element of the demo that surprised us the most was the framerate, which was significantly choppier than what we saw of the game when we visited Silicon Knights back in February. What changed since then?
DD: Several things actually. Epic has added significant technology to the Unreal Engine 3 as they try to adapt it to the 360 hardware, and we have as well. Unfortunately, there was not time to fully optimize the newest adaptations before E3. You guys saw it running in a better framerate when you visited Silicon Knights. We are confident that we will get the framerate back and well beyond the framerates that you first saw.
I guessed right for a change...