Note that I say "at a fundamental level." The truth is that, if you strip away all of the Vista features -- disable all the new services (Search, SuperFetch, MMCS), turn off all the new UI goodies (Aero, compositing), and terminate every noncritical process -- Windows Vista is still 40 percent slower than Windows XP. In fact, no matter what you try, you will never manage to tune Vista so that it performs like Windows XP on identical hardware. Never. It's simply not possible.
Why? Kernel complexity. Those extra 39 to 41 execution threads (96 to 97 versus 56 to 57 on XP) translate into a kind of performance anchor around Vista's neck, causing even a stripped-down, bare-bones implementation to perform at degradation levels that approach disabling a core or drop a gigabyte or two of memory. It's why XP is roughly 18 percent slower than Windows 2000 (due to its 22 additional kernel threads). And it's also why Windows 7 performs almost identically to Vista (no significant increase in kernel complexity -- yippee!).
In fact, if you look at the history of the NT kernel, and you map the increase in thread count against the decrease in performance (relative to the previous iteration), you come up with a nice little ratio that fits the data almost perfectly: For every additional execution thread added to the kernel, linear performance decreases by 1.0 to 1.2 percent.
Again, this isn't speculation. It isn't conjecture. It's an observation made over several generations of the NT OS, a convenient rule of thumb that has yet to be disproved. And I do hope that both our reader communities will accept my not-so-subtle challenge to disprove the kernel thread rule by downloading the free tools and resources of the Windows Sentinel project.