I have a system using an internal HD with 2 partitions, one being Mavericks and one being Lion. I was having some trouble and thought maybe the drive was the problem. This is an older MacPro. I figured it was likely the boot region of the drive in Mavericks because the system seemed to be bogging down during bootup. I took a copy of Scannerz and did a 0-20GB sweep of the drive, not the whole thing, to see if it found errors in the boot regions. It found none. I thought then that because when the OS boots it checks other drives and volumes maybe something was wrong with with the Lion partition, so I did a 0-20GB test on it too with Scannerz. Once again, nothing wrong. I decided to boot into Lion to see if I'm getting the same sort of delay. It didn't. It booted nearly twice as fast as Mavericks though. Once there I decided I might as well repeat my Scannerz tests on the 0-20G regions of both the boot volume (now Lion) and other volume (now the Mavericks volume). Both scans completed about 20% faster on Lion than they did on Mavericks. Why is Mavericks so much slower? Scannerz is a test tool for finding problems, not a performance tool, so when I have time I think I'll try the same stunt using Black Magic speed test. Anyone else have any comparisons of performance data between different OS versions. It seems to me that the more we go in time the more bogged down the performance is with every newer release of OS X.