there aren't any fast computers. Some computers are faster than other computers, but owning a faster computer merely lets you run a more advanced operating system and suite of software, and slows down the actual speed of the computer till it's the same as always. There's always the thought 'I could run, say, Windows 3 on this machine and it would fly', and you could, but there's a reason no-one uses Windows 3 any more, and it's Trumpet winsock (oh, the memories).
every new version of windows (I'm using windows as an example for the moment, but don't get hung up on it, we can and will apply this to other things) has features the last one didn't. I could be mostly happy with windows 2000, say. But Windows XP does this really nifty thing when you put a blank CDs in, it'll open an explorer window and you can drag files into it and burn them to CD, so you don't have to mess with awful 3rd party crap. Or when you plug a digital camera in it'll do the right thing, and again you don't have to mess with nasty 3rd party camera drivers. (yeah, yeah, stifling innovation, waffle waffle waffle. Have you ever used the obscenity Canon try to foist off on you as an interface?)
So on a decent machine with a CD burner that will take it, I'll install Windows XP. And so the more powerful machine will be slowed down by the overhead of trying to look like a huge boiled sweet. I'll live with this, because I get other benefits. But it's annoying.
It's not just Windows. Gnome 2, say, is slower than Gnome 1. But it's much nicer. Mac OS X is a bit slower than Mac OS 9 (a bit slower. Just a little bit). I have a mac laptop now that's 3 times the speed of the last laptop I had. It's about the same speed to use. Certain things have changed - I can scroll web pages now and not have to worry about redraw speed, which is nice, but the slowness has moved into other things. App-switching can be sluggish sometimes. Performing certain functions in apps can be slow. But it's so nice, and I'm sure not running linux on this thing, not after the pure slickness that is OS X, it would be unbearable. So my machine is the same speed as the last one.
(Anyone saying I need more memory is missing the point. There's >300 megs in here, linux would fly... I know MacOS wants memory, that's my point)