The myth
Dust makes your PC slower. The line comes up in every conversation about ageing machines, and the outcome is often even true. It is the explanation behind it that is wrong, and the explanation decides where you need to clean.
What actually happens
Dust does not brake a single calculation. What it does is hold up air: it clogs heatsinks, fans and dust filters, blocking exactly the parts that are supposed to carry the heat away. NVIDIA documents a threshold of around 85 degrees Celsius for most graphics cards: once the chip reaches it, the driver lowers the clock speed to keep the card from overheating, and it is this throttling that costs you frames.
So the path runs through temperature, not through the dust itself. Throttling is not a defect either, it is a protection mechanism: the card backs off before it takes damage.
The grain of truth
It is a big one: the outcome is real, a dusty machine often does get slower in practice. But only if you know the causal chain do you clean the right spot, and recognise the case where cleaning does nothing at all because the temperatures were fine to begin with.
What to do instead
Check the dust filters first: front, bottom and top. Properly placed filters catch dust before it ever enters the machine, and they can be cleaned. Intel recommends carefully cleaning the case with compressed air every few months, holding the fan blades in place while you do it so they do not spin and damage the motor.
Whether your card throttles at all is visible from its temperature under load. The cleaning guide and how to read those temperatures are on the optimisation page.