OS9 flies on a G4. The only thing that's slow is building the file system when it boots if you have a lot of files. I have one G4 with a couple of 1 TB drives and OS X installed also. With nearly a million files to index at boot OS 9 takes a couple of minutes to finish building the desktop. OS X saves the cached file system to disk and reloads it at boot, so it comes up much more quickly.