Every release had a ~2 factor in performance improvements between the public beta and Tiger (10.4). Performances degraded a bit with Leopard (but improved again with Snow Leopard). That’s why Tiger is a bit of a high water mark for PPC machines. It was very useable on a G3 from 1999, albeit with a more recent graphics card for Quartz Extreme and quite a bit of additional RAM over its initial specs.