Take a look at the Activity Monitor to see if it can shed light on the performance.
As mentioned above, just how much free space do you have on your boot volume? You should keep about 20GB to 25GB for that.
Have you run Disk Utility to make sure all your volumes ar ok? Repair them with DU. Boot from your installer dvd or recovery partition (you didn't say what OS you are running), or an another bootable volume, to repair your boot volume.
Do a smc reset.
Clone it and test on the newly formatted drive
A safe boot or two and clean up your browser cache
a new user account
lots of things but there is none that we can guess
and there are always a number of threads esp when a new OS and people upgrade
what apps, what you do, how you configure your work and storage
a good place to check as well: