MacOSX 1.0.0 - 10.3 was slow as hell.
Really? What are you basing this assertion on? 10.2 and 10.3 weren't very refined overall, but they were plenty quick with sufficient RAM and GPU, especially given the fact that the Quartz graphics system that they had developed way back in 2001 was (besides Amiga OS) the first mainstream OS to include graphics compositing capabilities to offload window manager rendering from the CPU to the GPU. Apple was way ahead on that, MS didn't even have Compositing working in Windows until Vista came out in '07.
10.6 was fast enough to run on the colored iMacs taht 10.1 could not, hence these users stayed on MacOS classic.
10.6 was the first OS X release that ditched Universal binary support and went Intel only, so no, it would not run on "colored iMacs" at all since they were PowerPC G3's. Hell, 10.5 wouldn't even install on the newer G4 iMacs without being forced to do so through OpenFirmware hacks or installer modifications. Running 10.5 was painful enough on those, it'd be relegated to novelty status on a G3.
Today WIndows 8 runs on 9 year old Pentium IV with ease.
Somebody correct me if I'm wrong, but isn't Win8 64-bit only? Is there even a such thing as a Pentium 4 capable of executing x64 code? My memory is a bit foggy on the P4's because AMD was trouncing Intel on price/performance back then, so I never had much experience with those space heaters. In any case, you'd have a hell of a hard time finding Win8 drivers for any system built around a P4, or an Athlon for that matter.
What you're saying about newer Mac OS X and other OS releases getting more efficient and faster is correct, but your historic examples are way off. Now get off my lawn!