It's not supposed to happen, but of course over time Macs start to slow down. Apple will tell you all day long that defrag is unnecessary, permissions are fine etc ... this site does a *really good* job of outlining common problems, the myths associated, proggies to sort yourself out and reasons why/why not. I was most impressed and found the data to be strong and accurate.
http://www.macattorney.com/ts.htmlThe single largest piece of data I got from it was to download the iDefrag demo and look at drive fragmentation as opposed to disk fragmentation. My machine has been steadily slowing down (gradually and not unworkable, but noticeable) over the last couple years. I have about 18% free on my hard primary system hard drive which should be enough. iDefrag told me something really interesting: my
free space was 99.8% fragmented. In other words, the machine was working it's ass off to swap my big 'ol memory pool into little chunks of hard drive. This is different than file defrag or optimization, which focuses on the load and seek time intra-file. iDefrag LOOKS LIKE it will do a nice job of adhering to the notion of the Hot Zone when optimizing and will move Metadata into the right spot on the drive, evicting stuff that doesn't belong there as well. I have no idea how long it's going to take to DF my 2T drive, so I won't be doing it till tonight.
Interestingly, it looks as though Mac's version of the HFS+ does a damn fine job of keeping
files defragged. I'm only at about 1.03% total fragmented which, given the age of this system since reinstall is damn good.
Anyhoo, it's a rather long read but is well worth it. Particularly if you ever need to triage OSX behavior.