One of the most important things about CPUs is speed.
The speed of CPU is measured in Hz, i.e., number of operations can be done per second.
Nowadays, we ususally use MHz(e.g. 300 MHz, 550 MHz) because the CPUs are too fast to measure in Hz.
Because of the extremely high speed of CPU, computers can do millions of operation in a very short time.
Heat is released when CPUs run. High temperature, however, decrease the performance of CPU. You, therefore, need a fan to cool down the CPU.
You can change the speed of CPU in BIOS of your computer. Some people change the speed of their CPU to a very high MHz. This can increase the speed of the computer but would make the CPU overload and overheat. Finally, the whole computer may break down. Therefore, it is not recommend to do so.
Proceed to : Performance of CPU