People confuse the snappyness of the OS GUI with speed of doing calculations.
I find OS 9.x to be plenty fast on old machines unless you are doing some major multitasking. OS 9.x has less overhead then OSX 10.3.x, so while the GUI might feel snappy using OSX the tasks in the background run slower to make up for it (same CPU and all). Also OS 9.x works better with less RAM then OSX does (load up a few apps and see). Classic mac OS is best for task switching, and kind of sucks for new multimedia (lack of codecs, baddly written java, old browsers).
I find OS 9.x to be plenty fast on old machines unless you are doing some major multitasking. OS 9.x has less overhead then OSX 10.3.x, so while the GUI might feel snappy using OSX the tasks in the background run slower to make up for it (same CPU and all). Also OS 9.x works better with less RAM then OSX does (load up a few apps and see). Classic mac OS is best for task switching, and kind of sucks for new multimedia (lack of codecs, baddly written java, old browsers).