Well think about it.
Dust is ever present in the atmosphere, its a natural thing.
Computer fans on the case suck in air and everything in that air, which includes dust particles.
Dust particles settle down when the system is not in use. Usually they'll settle on the bottom of the case but often end up on top of any peripherial cards you have - graphics cards, modems, etc.
Particles are also attracted to the heatsink fan on top of the processor - because it itself is sucking in air to blow onto the processor. Therefore they accumulate on the heatsink fan as well.
The rest from here is natural physics: A huge collection of dust particles which get stuck together (hence "dust bunnies") form on and behind the heatsink fan. They generate friction on the fan's rotation, which itself generates heat.
Therefore a clogged up fan eventually can't blow enough cold air downwards and will either stop spinning altogether due to being clogged up OR will start blowings its own friction-generated heat downwards as well as any cool air that gets through.
The result: Overheating, not good for processor heatsinks.
As for overall speed increase: Computers don't like being hot. Most modern processors will throttle their speeds to run slower if they're running too hot in an attempt to cool themselves down before they reach any cut-off point set in the BIOS.
Edited by Neil Jones, 07 November 2005 - 05:44 AM.