My mac OS X used to be pretty decent in speed, and I dont really use it for much, it has music that plays 24/7 and a web browser (Firefox) with about 5-7 tabs open. I don't know much about macs, but I don't really use this one for much and I have no idea why its slowing down so much. Whenever I try to click something it takes like 20-30seconds to respond.
Don't know about macs, but FF on windows is a memory hog. Try using Camino and restarting it once in a while. You should probably try to defragment the drive (also don't know if macs can be defraged, but it is worth a try).
How old is your mac? I have a G3 500 ibook that gets slow at some times just because of all of the cached files that clog it up. You may want to try a program called Onyx. It will restore your mac to running at normal speeds again by repairing the permissions, clearing all of the caches and performing the maintanance scripts.
Just shoot on over to Version tracker and download it. It's Free
You might have alot of programs open at one time. you can see if they are open by looking at the quick lunch to see if they are active. look for the little arrow in the bottom. if you see it that program is running? exit by holding the mouse button on the icon. you should get a quit opption
Hanyman brings up a good point. What is the model of the computer, how much ram does it have and what is the operating system. You could also check your activity monitor.app and select "all processes" and list by CPU. That will show running applications (even background ones) that are running in order of how much system resources it is using. You might find that you have 3rd party programs running that you didnt even know about.
Another thing... in iTunes have a look at your playback settings in the prefs. I think these when active can increase your overhead.