GCSEPaper 1Systems ArchitectureCPU performance
1.1.2

CPU performance

How clock speed, cache size and the number of cores affect the performance of a CPU, individually and in combination.

Learning Objectives

  • 1Describe how clock speed affects CPU performance
  • 2Describe how cache size affects CPU performance
  • 3Describe how the number of cores affects CPU performance
  • 4Explain how changing these characteristics, individually or in combination, affects system performance

Clock speed

The clock speed is how many cycles the CPU can carry out each second, measured in hertz (Hz). Modern CPUs are measured in gigahertz (GHz), which is billions of cycles per second. A higher clock speed means the CPU can carry out more instructions each second, so it works faster.
Clock speed is the number of clock cycles a CPU can carry out per second, measured in hertz (Hz). A CPU running at 3 GHz completes around three billion cycles per second. Increasing the clock speed means more instructions can be processed each second, improving performance.
Clock speed is measured in hertz and determines how many instructions can be processed per second. Increasing the clock speed increases the number of clock cycles completed each second, so more instructions are executed in the same amount of time, improving performance.
The same one second of time1 GHzAbout 1 billion cycles per second3 GHzAbout 3 billion cycles per secondMore cycles per second = more instructions processed

Think of it like...

Clock speed is like how fast a worker completes tasks. A faster worker gets through more tasks in the same hour.

Cache size

Cache is a small amount of very fast memory inside the CPU. It stores data and instructions the CPU uses a lot, keeping them close by. A bigger cache can hold more of this frequently used data, so the CPU spends less time waiting for data from slower RAM, making it faster.
Cache is fast memory that stores frequently used data and instructions. A larger cache can store more frequently used data close to the CPU, so the CPU needs to fetch data from slower RAM less often. This reduces waiting time and improves performance.
Increasing the cache size allows more frequently used data and instructions to be stored close to the CPU. This means the CPU has to fetch data from slower main memory (RAM) less often, reducing delays and improving performance.
CPUVery fastCacheSmall, fast memorySlowerRAMLarger, slower memory

Think of it like...

Cache is like keeping the tools you use most on your desk, instead of walking to the cupboard every single time you need one.

Number of cores

A core is like a CPU within the CPU. Each core can carry out the fetch-execute cycle on its own. A CPU with more cores can work on more instructions at the same time, so it can do more at once.
A core is a processing unit within the CPU, and each one can run the fetch-execute cycle independently. A CPU with more cores can process multiple instructions at the same time (in parallel), improving performance, especially when multitasking or running software designed to use multiple cores.
Each core can execute instructions independently. Multiple cores allow instructions to be processed in parallel, so more can be completed in a given time. The size of the benefit depends on whether the software is written to make use of several cores.
Single coreCoreInstructionOne instruction at a timeFour coresCoreInstructionCoreInstructionCoreInstructionCoreInstructionFour instructions in parallel

Think of it like...

One core is one chef in the kitchen. Four cores is four chefs, they can cook four dishes at once, as long as there are enough separate dishes to prepare.

Putting the factors together

No single factor decides how fast a CPU is, they work together. A CPU with a high clock speed, a large cache and several cores will usually outperform one that is only strong in one area. In the exam, if you are asked how to improve performance, mention more than one factor.
CPU performance depends on clock speed, cache size and the number of cores both individually and in combination. Improving one factor helps, but the biggest improvements usually come from improving several together. For example, more cores combined with a larger cache lets the CPU process more instructions in parallel without waiting as often for data.
The three characteristics affect performance individually and in combination. Increasing clock speed raises the number of cycles per second; increasing cache size reduces how often the CPU waits for data from RAM; adding cores allows parallel processing. A balanced improvement across all three typically gives a greater overall performance gain than improving any single factor on its own.

Exam Phrasing: Use These Exact Phrases

  • ✓Clock speed is measured in hertz and determines how many instructions can be processed per second
  • ✓A higher clock speed means more instructions can be processed each second
  • ✓A larger cache means the CPU needs to fetch data from RAM less often
  • ✓Multiple cores allow instructions to be processed in parallel
  • ✓The characteristics affect performance both individually and in combination
  • 💡If asked how to improve CPU performance, mention more than one factor: clock speed, cache size and number of cores.
  • 💡Don't just state a factor: explain the effect. E.g. "a larger cache means fewer slow fetches from RAM, so the CPU waits less".
  • 💡Remember more cores only give their full benefit if the software is able to use them.
  • 💡Clock speed is measured in hertz (Hz), and in gigahertz (GHz) for modern CPUs.

Wrong:

More GHz always means a faster computer

Right:

Clock speed is one factor; cache size and number of cores also affect performance

Why it matters: Examiners want you to show that performance depends on several factors, not just clock speed.

Wrong:

More cores always doubles or triples the speed

Right:

More cores allow parallel processing, but the benefit depends on the software being able to use them

Why it matters: Overstating the effect of adding cores is a common error.

Wrong:

Cache is the same as RAM

Right:

Cache is smaller and much faster than RAM, and sits inside or very close to the CPU

Why it matters: Confusing cache with main memory loses marks.

Question:

A laptop runs slowly when several programs are open at once. Explain how increasing the number of cores could improve its performance. [3]

💡 Attempt this before revealing the answer, practice makes perfect!

Model Answer:

Each core can carry out the fetch-execute cycle independently, so a CPU with more cores can process more than one instruction at the same time, in parallel. With several programs open, the work can be shared across the cores, so more instructions are completed in a given time and the laptop feels faster. This benefit is greatest when the software is able to make use of multiple cores.

Mark Scheme:

  • 1Each core can process instructions independently / in parallel
  • 2More instructions can be completed in a given time (improving multitasking)
  • 3The benefit depends on the software being able to use multiple cores

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Exam-style questions. Have a go, then reveal the mark scheme to check your answer.

1.State what is meant by the clock speed of a CPU.

[1]

2.Explain how increasing the size of the cache can improve the performance of a CPU.

[3]

3.A CPU has four cores. Explain how having multiple cores can improve performance.

[3]

4.Two computers have CPUs with the same clock speed. Explain how one computer could still perform better than the other.

[4]

5.A computer is being upgraded. Explain how increasing both the clock speed and the cache size could improve its performance.

[4]

6.Two laptops cost the same. Laptop A has a 3.6 GHz dual-core CPU with 2 MB of cache. Laptop B has a 2.4 GHz quad-core CPU with 8 MB of cache. Discuss which laptop is likely to be better for video editing.

[8]

Extended response · marked in levels, not by counting points

2025Question paperMark schemeExaminers' report
2024Question paperMark schemeExaminers' report
2023Question paperMark schemeExaminers' report
2022Question paperMark schemeExaminers' report

The examiners' reports are the most useful and least read of the three: they say what candidates actually got wrong each year. All files are hosted by OCR.

Past-paper questions grouped by topic

The same OCR questions, sorted by topic rather than by year (via Physics & Maths Tutor).

2024
Paper 1

Examiners noted that answers about CPU characteristics were often too vague. Writing "clock" or "core" instead of "clock speed" and "number of cores" was considered too ambiguous to earn the mark, so always name the full characteristic precisely.

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