Clock speed is a measure of how quickly a computer completes basic computations and operations. It is measured as a frequency in hertz, and most commonly refers to the speed of the computer's CPU, or Central Processing Unit. Since the frequency most clock speed measures is very high, the terms megahertz and gigahertz are used. A megahertz is one-million cycles per second, while a gigahertz is one-billion cycles per second. So a computer with a clock speed of 800MHz is running 800,000,000 cycles per second, while a 2.4GHz computer is running 2,400,000,000 cycles per second.

How clock speed should be used as a benchmark of a computer's speed is a matter of some contention, though most chip makers appear to be drifting towards the conclusion that clock speed should be abandoned as the primary value given. The problem comes from the fact that, although clock speed works as a fairly reliable indicator of how one of a company's chips stacks up against another of their chips, it is a poor indicator of how that chip might compete against a different company's chipset. One reason clock speed of the CPU isn't such a reliable test of overall computer speed is that many other factors come into play. The amount of RAM a computer has, the clock speed of that RAM, the clock speed of the front-side bus, and the cache size all play significant roles in determining overall performance.

When comparing one Intel Pentium chip to another Pentium chip, for example, the clock speed is a fairly good indicator of an improvement in speed. An 800Mhz Pentium computer will perform most processor-based tasks roughly twice as quickly as a 400MHz Pentium computer. When comparing a different company's processors, however, the story changes. If we look at both a Pentium chip and an AMD chip, for example, we find that the AMD tends to perform most tasks more quickly than a comparable Pentium. A 1.8GHz AMD chip significantly outperforms a Pentium 1.8Ghz chip, instead performing near the speed of a Pentium 2.2Ghz.

For this reason, AMD stopped listing their clock speed as a primary method of advertising on their computers, instead associating a number with the computer meant to show its performance in comparison to a Pentium. The AMD Athlon 64 3000, for example, has a clock speed of only 1.8GHz, but AMD determined it to be roughly comparable to a Pentium 4 at 3GHz. Intel itself has also begun to move away from the clock speed model of advertising, mostly due to their introduction of a laptop-oriented M line, which has much lower clock speeds to optimize portable performance. By sticking to a clock speed model, Intel made its Pentium-M computers look like they were slow and weak compared to their Pentium 4 models.

While clock speed can still give a general idea of computing power, it is more often recommended these days to look at some sort of trusted benchmark as a way to compare computers. Looking at benchmarks of how various processors handled tasks you care about will give a clearer picture of how the computer will perform for you. One computer might be significantly faster than another at transforming images in a graphic design program, for example, but slower at video-intensive games. This paradigm of more task-specific computer ratings, rather than a focus exclusively on clock speed, ultimately offers consumers a better understanding of what sort of product they can expect.