Computer users expect their systems to work well at all times, but unfortunately this isn’t always the case. If your system becomes slow, there certainly is something you can do about it. This article will help you understand what’s happening on the system, whether it’s the computer in front of you or a system you’re accessing remotely. Naturally, I presume you’re running Linux, and the tools described here are Linux tools. If you’re on some other weird system ( ;) ), your mileage may vary.
When we say the system is slow we mean that it isn’t responding to our input in a reasonable time, or taking too long to complete a task. This can happen when there is *another* program using too many system resources, starving *your* program of resources, causing it to run slowly
There are three common ways in which this can happen, and all three of these scenarios can be equally crippling and put your system into a state where it seems to be frozen. None of these situations are harmful to your system (ie. they make it slow until the problem is resolved, but they don’t damage anything).
Click for the Full Story
http://hehe2.net/linuxhowto/3-reasons-why-your-system-might-be-slow/