Redhat probably had more services installed and running as opposed to slack. Possibly, Rhat did not enable the best DMA or UDMA on your HD. It is more likely distro dependant, however, I get Redhat working ultra fast on my 2 GHz, and decent performance on my son's P 233. It comes down to, usually, disabling/stopping services, getting the best video driver you can and turning off uneeded eye candy. GNOME is faster than KDE while KDE is faster than GNOME. GNOME loads quicker, but needs to load more on app start, whereas KDE loads slower, but keeps more libraries in mem (is more of a hog that way), and apps start/restart quicker. On a machine is less memory, GNOME will appear faster. On a system with memory to spare, KDE will seem faster in desktop operation.
However, all things being equal, they would be the same.