I just upgraded my G4 tower (867Mhz, 1.25 GB), to Leopard from Tiger and it's performance is completely shot. Everything moves like molasses, it takes ten to twelve minutes just to boot and even blue-screens a bit, the mouse skitters around like a drunken tse-tse fly and web pages take forever to load. The behaviour is exactly the same in fact as when the Spotlight indexing happens for the first time, but the indexing has already completed. All the typical causes like APE and DivX have been eliminated as possibilities but the entire computer still acts the same. It turns out that a probable cause for my problem is the fact that the main hard drive has failed it's "SMART" test. I already new that the SMART status was failing when I was in Tiger, but I forgot about it since there was literally no effect. About four months ago I noticed that the drive had failed the test, but since I have daily bootable backups and I was not experiencing any trouble with the drive at all, I let it go. I am getting a new machine in a couple of weeks anyway. In Leopard however, the drive with the failed test shows up in bright red type when examined with the Disk Utility, and reports a "fatal error" on the drive. This causes Spotlight indexing to not only take forever (16 hours the first time on a 60 Gig drive), it also randomly fails and has to re-index itself all over again. I have allowed Spotlight to completely re-index all the drives three times already. At least 80% of the time I am using the computer, Spotlight is trying to re-index, but even those times when it's not, the user experience is the same. Choppy, severely slow, almost unusable computing. The point is, what was an almost inconsequential disk warning under Tiger that one has to use the system profiler to even know about, equates to a disk that can hardly be used at all with Leopard! It seems that Leopard is much less forgiving of your hard disks than Tiger was. The only solution seems to be to replace the hard drive. I thought it worthwhile to post this as most average users will not ever even check on the SMART status of the hard drive or even know how to. This could be a reason for very poor Leopard performance on a computer that had previously no known (or at least visible) problems.