If you think your MacBook or notebook is slower than when you bought it you might be right. If you have an Intel CPU a dirty fan can make your computer slower because the CPU gets too hot and thus the CPU frequency is reduced. You can examine it by installing the Intel Power Gadget. This is how it looks if the fan cannot cool the CPU enough so that it stays at 100°C:
What you can see here is that if the CPU is used much (“Utilization”) the temperature reaches 100°C. And to decrease the temperature the CPU frequency is reduced, making the CPU slower. In this case it had only 1,7 Ghz in the end.
After cleaning the fans of the MacBook the cooling worked better and it became much faster again.