Probably because the machines in use were either older machines or prototypes, it was mentioned repeatedly that they're working with device manufacturers to improve any latency issues because they are infintely more problematic when dealing with a touch device than they are with mouse/keyboard combos. I'm also pretty sure there will be lots more works done on this specifically before release.
But that was part of Sinofsky's point in the keynote: with 450 million Windows 8 capable machines out there we have a lot of customers for Metro apps. if it doesn't perform well on those older machines that notion of Sinofsy's is a falicy. They said the same thing about startup time and power management with Windows 7 and there weren't any huge strides forward there either. They boast of all of the power usage improvements they made in the Big Picture sessions but the tablet they handed out gets 3 to 4 hours battery life at best being miserly with turining off all radios and using power saver mode exclusively. You can say "well that's because you were doing x, y, or z" but in the end it's no match for the iPad nor Android tablets. You could also argue "that's not a fair comparison; wait until the ARM slates arrive and then compare." To which I counter with "and throw out your app capatibility with it; your options on ARM will be limited to media, social, internet, and fart apps for some time." Microsoft is creating a nice big ditch for themselves and us developers to try and crawl out of with Windows 8. Their only upside here is that PCs will continue to sell and have Windows installed on them. The WinRT/Metro side of Windows 8 is a long shot at best.