I have a theory.

I think microsoft deliberately adds delays/slowdowns/unoptimized code into windows.

Then each release they fix some of it - and voila it's faster than the one before it.

With vista they finally ran out of delays that they could remove, and that's why it's so slow. So they figured, if it's slow, let go all the way, and added in a new batch of slowdowns.

And right on schedule Windows 7 is faster than the one before it.