Clock speed is the rate at which a processor can complete a processing cycle. It is typically measured in megahertz or gigahertz. One megahertz is equal to one million cycles per second, while one gigahertz equals one billion cycles per second. This means a 1.8 GHz processor has twice the clock speed of a 900 MHz processor.

However, it is important to note that a 1.8 GHz CPU is not necessarily twice as fast as a 900 MHz CPU. This is because different processors often use different architectures. For example, one processor may require more clock cycles to complete a multiplication instruction than another processor. If the 1.8 GHz CPU can complete a multiplication instruction in 4 cycles, while the 900 MHz CPU takes 7 cycles, the 1.8 GHz processor will be more than twice as fast as the 900 MHz processor. Conversely, if the 1.8 GHz processor takes more cycles to perform the instruction, it will be less than 2x as fast as the 900 MHz processor.

Other factors, such as a computer's bus speed, cache size, speed of the RAM, and hard drive speed also contribute to the overall performance of the machine. Therefore, while the processor's clock speed is a significant indicator of how fast a computer is, it is not the only factor that matters.