I must interject and lower that "good performance" barrier down to 400MHz for a G4. I have 640 MB of RAM installed, and OS X performs "nicely." Yes, I'd love more speed, but who wouldn't? Who wouldn't love more speed in OS 9? I agree, OS X can be a bit taxing at times on my machine, but nothing that's going to make me pull my hair out.
I consider myself a "power-user," as well, so I am GREATLY concerned with the overall perceived speed of a system. I can fly through OS 9 windows like a bat out of hell, usually making people's heads spin if they try to watch. OS X -- not so much, but you learn the fastest route to something one way or another, and I can get to a nested sub-folder 30 levels deep just as fast in OS X as I can in OS 9.
Yes -- the finder seems a bit sluggish. But seriously, folks, who needs to resize and reposition windows every 5 seconds? You can just as quickly hide the foremost app and do your finder stuff, then unhide... you can even do this quickly in OS 9. I understand that SOME resizing and repositioning may be necessary, but if you do this in excess, then hell yeah you'll notice some slowdown and might just possibly need to reconsider how you work if you're doing this that much!
Point: OS X is different from OS 9. I've started working in OS 9 again frequently, just because a few programs haven't been ported yet, and it was a very similar experience from going from OS 9 to OS X the first time -- except backwards. I had to remember foreign key commands I hadn't used in ages and had to navigate the desktop and folders a bit differently. I think OS X has made the transition from OS 9 to OS X pretty painless, but there WILL be some differences. Think about it: if Jobs and Co. had made OS X just like OS 9 with the exception of the UNIX underpinnings, we'd all be screaming that he didn't think different enough. He kicked everything up a notch -- graphics, animations, usability, stability -- everything. Give it time. Remember the headache OS 8 was? Well, it paved the way for 9 and from what I can see, OS 9 has a TON of support here... in terms of speed, usability, etc... OS 8 didn't run as fast as OS 7 on the same machine -- similar to OS 9 and OS X.
I'm just as critical as the next guy/gal, but I tend to be an optimist and can see great things in store for OS X. I'm always waiting for the next upgrade to see what's been added or improved and haven't been let down yet.
Props to OS X.