This is a phenomenon that I have experienced on a Blue & White G3 and a Gigabit G4, and in OS 9 and OS X: Whenever the hard disk spins down, the mac goes really slow even after it spins back up. I have to set the Energy Saver settings to keep the HD and computer from going to sleep (same result when computer goes to sleep). The problem is only fixed after I restart the machine. Same result with different hard drives as far as I can tell. Does anybody else have any experience with this or any additional explanation?