I just read up on why apple computers get slow. Turns out that the bottom half of the hard drive is very slow to access for the computer so you have to try to keep it only half full (this is not a problem in the PC world). ALso, it says a mac/apple will slow down over time due to all the OS upgrades.
It says the only real solution is to copy all your data to an external drive, then erase your hard drive, then copy all the data back to your hard drive. This moves your data higher on the hard drive (so its not on the bottom where its slower to access). But you have to have only about 50% full hard drive for it to make a difference.
I got all that from an article on Apple's website:
https://discussions.apple.com/docs/DOC-4032