The thing that most people fail to understand is that new features don't just magically appear. They take CPU cycles, and disk accesses. The OS does more, so yes, it will be slower than previous versions.
Imagine if you were doing a job, then your boss put 2x as much work on your desk. It would take you longer to finish, would it not?
You can make windows faster, but in order to do that, you have to disable all the stuff that makes the new version.. well.. the new version.