What makes a computer fast: clock speed, cores, bus width and cache

A computer performs well when it finishes work quickly — when instructions are carried out without the processor sitting idle waiting for memory.

Four factors decide most of that, and the syllabus names them:

• Clock speed. More cycles per second means more steps per second, so an instruction takes less real time. It is usually quoted in GHz.
• Number of cores. A core is a complete processor. Two cores can execute two instruction streams simultaneously, so a machine with more cores can run several tasks at once — if the software can share the work out.
• Bus width. A wider data bus moves more bits per transfer, so more data arrives with each trip: a 64-bit bus delivers eight times as much as an 8-bit bus.
• Cache memory. Cache is fast memory close to the CPU (the IAS you met earlier). A bigger cache keeps more of the current working set nearby, cutting waits for main memory — so the CPU spends its cycles working instead of waiting.
• Processor type. A newer design may do more useful work in a single cycle than an older one at the same clock speed, so GHz alone never tells the whole story.