Interesting link comparing speed of different versions of OS X. The 10.1.3 results might be invalid, though, as their testbed has grown by 512MB since the 10.1.2 benches. (See site to know what I'm talking about.)
I've been usingm OS X since 10.0.3, I was running that on a Pismo 500. It was horrendously slow. But I loved the stability, and i soon found I couldn't live in 9 any more. Right when the 10.1 upgrade came out, i sold my Powerbook and bought a G4 533, Now with 10.1.3 I don't have any real big speed issues, other than i want a faster card than my ati 16 meg'er(but thats a different story) The only speed problems i ever have is that wonderous spinning color wheel when i go to some websites....but it think that is a problem with IE, It is the only program i ever have to force quit.....Once that gets fixed i will be set!