3.3 CPU Performance Trainer
Three factors that affect CPU performance — cores, clock speed, and cache. Cambridge tests these in comparison questions every session.
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What do I need to know before I start?
CPU performance is how quickly the CPU can execute instructions. Three factors directly affect it: clock speed (how many FDE cycles run per second), number of cores (how many instructions can be executed at the same time), and cache size (how quickly frequently used data can be accessed). Cambridge tests all three every year, and each needs its own exact wording — vague answers like "it makes it faster" score nothing.
By the end of 3.3 you can…
| Objective | Why it matters |
|---|---|
| Define clock speed using Cambridge's exact phrasing | s25 mark scheme (3 marks): "the clock speed is the number of FDE cycles performed in a second". Each pulse = one FDE cycle. |
| Explain how more cores improve performance | s25 mark scheme: "each core executes one instruction at a time — e.g. 4 cores → 4 instructions simultaneously". |
| Explain how more cache improves performance | s24 mark scheme (2 marks): more cache → more frequently used data/instructions accessed quickly instead of slower RAM. |
| Compare two or three CPUs and justify the fastest | s25 Q3c — a 3-mark comparison giving the fastest CPU with two justified reasons. |
| Avoid the "clock speed = simultaneous cycles" trap | s25 examiner report: clock speed affects cycles per second, not parallelism. Cores give parallelism. |
| Know clock speed is a measurement, not a component | s25 examiner report: clock speed is not a physical part of the CPU. |
Cambridge-approved terms
The number of FDE cycles performed per second, measured in GHz. Higher clock speed → more cycles per second.
A processing unit within the CPU that can fetch, decode and execute instructions. More cores → more instructions simultaneously.
Small, very fast memory inside the CPU that stores frequently used data and instructions — faster to reach than RAM.
The unit of clock speed. 1 GHz = one billion clock pulses (FDE cycles) per second.
Main memory. Larger and slower than cache; the CPU falls back to RAM when data isn't in the cache.
The key word for cores: multiple cores execute multiple instructions at the same time — this is parallelism, not clock speed.
The three performance factors
| Factor | How it affects performance | Cambridge mark-scheme wording |
|---|---|---|
| Clock speed | Each clock pulse triggers one FDE cycle. Higher clock speed → more FDE cycles per second → more instructions processed per second. | "The clock speed is the number of FDE cycles performed in a second." (s25) |
| Number of cores | Each core can run its own FDE cycle. More cores → more instructions executed at the same time — if the software supports it. | "Each core executes one instruction at a time… 4 instructions executed simultaneously." (s25) |
| Cache size | Larger cache → more frequently used data/instructions held close to the CPU → fewer slow trips to RAM. | "More cache means the processor can access frequently used data/instructions faster instead of accessing slower RAM." (s24) |
Traps that cost marks every session
s25 examiner report: wrong. Clock speed means more cycles per second, not in parallel. Parallelism comes from cores.
Only if the software can use multiple cores. Say "each core executes one instruction simultaneously", not just "faster".
s25 examiner report: it's a measurement (cycles per second), not a physical part of the CPU.
Cache is inside the CPU — much smaller but much faster than RAM, holding frequently used data/instructions.
Every factor needs a mechanism: cycles per second, simultaneous execution, or fewer slow RAM accesses.
GHz measures speed (cycles per second), not capacity. Cache and RAM are measured in bytes.
Check yourself — tap to reveal
Answer in your head, then tap to see the model answer. Aim for 5 of 6 before moving to Learn.
✅ Ready for Learn?
Before moving on, make sure you can answer these without notes:
- Define clock speed, cores and cache — each in exam wording.
- How does each of the three factors improve performance?
- What's the difference between clock speed and cores?
- Why is cache faster than RAM?
- How would you justify which of two CPUs is faster?
Three performance factors
Cores
A core contains all components for one FDE cycle. More cores = more instructions processed at the same time. Dual core = 2 FDE cycles simultaneously. Quad core = 4.
Clock speed
The number of FDE cycles per second. Measured in GHz (gigahertz). 1 GHz = 1 billion cycles/second. Higher clock speed = more cycles per second.
Cache
Storage built into the CPU that stores frequently used data and instructions. Faster than fetching from RAM. Larger cache = less waiting.
CPU comparison (Mar 2025 exam style)
This exact format appeared in the Mar 2025 paper. Which executes more instructions simultaneously?
Computer A
Quad-core (4)
Computer B
Dual-core (2)
Computer C
Single-core (1)
Press a button to see the answer.
Why each factor matters
| Factor | How it improves performance | Exam keyword |
|---|---|---|
| More cores | More FDE cycles at the same time / multitasking | "simultaneously" |
| Higher clock speed | More FDE cycles per second | "cycles per second" |
| Larger cache | Less time fetching from RAM / frequently used data closer | "frequently used" |
Quick check
Cache stores:
Activity 1 — Which Factor?
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Activity 2 — Spot the Mistake
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Activity 3 — Compare CPUs
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Adaptive practice
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Cambridge-style questions
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Review quiz
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Exam traps
Memory triggers
🧠 "Cores = at the same time"
More cores = more instructions processed simultaneously. Think of multiple workers on one job.
🧠 "Clock = per second"
Clock speed is about how many cycles happen each second. Higher GHz = more cycles.
🧠 "Cache = close and frequent"
Cache keeps frequently used data close to the CPU. No need to go all the way to RAM.