How does dust affect a computer?

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.

While a rare occurrence, dust can cause shorts. Mostly it will screw up your fans and coat heatsinks, which negatively affects heat dissipation.

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.