2000 has a few (well... two that I can think of) disadvantages:
Less mutimedia support
No whizzy memory process stuff thats in XP SP2
Other than that... well not much I guess.
I would suppose it all comes down to your ability to tune, configure and tweak the install for the hardware that you have.
I'm surpirsed this thread hasn't errupted into a rather large arguement!
I use both OSs (one at work, one at home) and the speed is roughly the same (apart from -as mentioned- the startup and shutdown processes). I think nowadays that if your hardware is fairly good, you can run the OS at it's theorical maximum speed. What I mean is, browse around and change some settings here and there etc.
Of course this changes when we start to launch applications. Here is where you could argue one OS is faster than the other however that would be down to hardware. Of course faster hardware, faster experience.
Now I have confused myself so I'm going away LOL