It is generally known that dust makes computers slower, and that cleaning out the dust regularly can make your computer run better.
What I have not seen, is why this happens. From what I know dust doesn't stop electrons from getting from one side of the computer to the other. Does the dust only clog up the fans, making the computer hotter, thus making the computer slower? Or is there some other more mysterious force at work here?
Clogging up the fan is one side effect. It is also possible that dust can cause a short-circuit because it conducts electricity. Dust can also inhibit airflow, which prevents heat from being properly vented.
And once the heat goes up your CPU may throttle down to compensate. At least mine seemed to...cleaned it out and over all temp went down and perceived performance went up...never measured clock speed...maybe next time.
On more than one occasion I have "fixed" a computer that would not power on or wouldn't give any video output by simply vacuuming out all the dust. It can't be a heat issue when the computer is just turned on and doesn't do anything. I think that hair is probably the most likely culprit of the short-circuiting.