Microsoft claims that IE8 is faster than Chrome and Firefox. The question is if those results are because IE8 is actually faster, or if it's just that most sites are designed to work as smoothly as possible in IE. IE is the dominant browser, after all. …
Since there are even sites that are have optimizations to work smoothly in Firefox, I wouldn't be surprised if IE has the benefit of being the first browser sites are designed to work with as well.
So maybe Microsoft is actually comparing apples and oranges here?
They didn't include the latest available version of Firefox and Chrome either (although, admittedly, they are not final versions). Opera was also left out. And while pure rendering speed is a big part of the browsing experience, that won't help much without a snappy user interface which gets you where you want to go. That, and time-saving features, of which Opera happens to have a few 🙂