Measure of speed
A CPU's performance is measured by the number of instructions it can process per second. When referencing CPU speed, the megahertz (MHz) rating, or clock speed, is in fact monitoring the "heart-rate" of the computer, or the steady pulses that energise the circuits and process information. All CPUs are driven by a clock signal generated by a component on the PC's motherboard. The first CPUs ran with a clock speed of only 4.77 MHz (4.77 million cycles per second) — today, clock speeds of more than 800MHz, or 800 million cycles per second, are the norm, with high-end systems running at 1GHz (1 billion cycles per second) or higher.

On-chip cache
Each time the memory is accessed, CPU activity is suspended. This effect can be reduced considerably if a small "cache" of memory is installed on the processor itself. This cache stores the most recently used data and/or instructions and, with 16 or 32 kilobytes of memory, can work just as fast as the processor itself. If any of the information held in the cache is required again, it can be accessed directly from the cache rather than taking the slower option of obtaining it from the Random Access Memory (RAM).