A mac is no fun when it starts to slow down. Nobody wants to use their computer when booting takes 5 minutes, just opening a browser takes a couple of more minutes and when multi-tasking is almost unthinkable.
But, if your mac has slowed down and slowed down rather suddenly, it could be because of a problem that can quite easily be fixed. All you need to do is identify the problem. Here is a very good resource that talks about why is my mac so slow all of a sudden. By reading that article, you will be able to figure out what the problem is. The most common way to fix a mac that has slowed down very suddenly is to first find out what app is slowing down your mac by looking at your Mac’s activity monitor. In activity monitor, you will find how much memory and CPU load your mac is using in % terms. If any app is using more than 10% of memory or CPU, it is hogging your Mac and probably slowing it down severely.
Sometimes, a fix is as simple as just closing that app, evening if it means that you have to force quit it, and then restarting the app. You can also try to update that app to a more recent version or simply uninstall and reinstall it. As a last resort, you can also try to find and download and use an alternate app, one that isn’t very resource hungry.Why is my mac so slow? Maybe this is why! Address this problem and it might speed right back up!
Besides app that are very resource hungry, a full hard disk or OS X partition can also severely slow down your Mac. If this is indeed the problem, all you need to do is move out some data from the OS X partition out to an external hard disk. Or you can simply delete unwanted data. Either way, freeing up even 5 GB of space can sometimes make a tremendous difference.