CPU Performance Factors

J277 · 1.1 Systems Architecture · GCSE Computer Science

Component 01

Three Factors That Affect CPU Performance

OCR J277 specifies exactly three factors. Learn these precisely — exam questions are direct.

Clock Speed

Measured in GHz (gigahertz). Each tick of the clock allows the CPU to carry out one or more operations.

  • ▸ Higher GHz = more cycles per second
  • ▸ More cycles = more instructions processed
  • ▸ A 4 GHz CPU runs twice as many cycles as a 2 GHz one

Trade-off

Higher clock speed generates more heat and uses more power.

Number of Cores

A core is an independent processing unit within the CPU. Multi-core CPUs can process multiple tasks simultaneously.

  • ▸ Dual-core = 2 independent processors
  • ▸ Allows parallel processing
  • ▸ Each core handles a different task/thread

Trade-off

More cores = higher cost and power consumption. Only helps if software is multi-threaded.

Cache Size

Cache is a small, very fast memory built into (or close to) the CPU. It stores frequently used data so the CPU doesn't have to wait for slower RAM.

  • ▸ L1 cache — smallest, fastest, on-chip
  • ▸ L2 cache — larger, slightly slower
  • ▸ More cache = fewer slow RAM fetches

Trade-off

Cache is expensive to manufacture — more cache = significantly higher chip cost.

Memory Speed Hierarchy

Understanding why cache matters — the CPU must wait if data isn't in cache.

Registers
Fastest · Smallest (bytes) · Inside CPU
L1 Cache
Very Fast · ~32–64 KB · On-chip
L2 Cache
Fast · ~256 KB–2 MB · On/near chip
RAM
Moderate · GBs · On motherboard
Storage
Slow · TBs · HDD/SSD

Width represents relative speed. Larger cache means the CPU finds data faster, without waiting for RAM.

CPU Spec Comparator

Interactive

Adjust the sliders for each CPU and watch the performance scores update in real time.

CPU A

Clock Speed 2.0 GHz
1 GHz5 GHz
Cores 2
18
Cache 4 MB
2 MB32 MB

Score

0

CPU B

Clock Speed 3.2 GHz
1 GHz5 GHz
Cores 4
18
Cache 8 MB
2 MB32 MB

Score

0

CPU C

Clock Speed 4.5 GHz
1 GHz5 GHz
Cores 2
18
Cache 16 MB
2 MB32 MB

Score

0

CPU A0
CPU B0
CPU C0

Score = Clock Speed × Cores × log₂(Cache). Higher is better.

Which CPU is Best?

Quiz

For each scenario, choose the most suitable CPU. Think about which factor matters most.

Scenario 1

A video editor needs to render multiple video streams at the same time. Which single factor is most important?

Scenario 2

A gaming CPU needs to execute a single-threaded game engine as fast as possible. Which factor has the most direct impact?

Scenario 3

A database server repeatedly accesses the same small set of frequently-used records. Which factor would most reduce waiting time?

Key Terms

GHz

Gigahertz — unit of clock speed; 1 GHz = 1 billion cycles per second.

Core

An independent processing unit within a CPU; each core can execute instructions separately.

Cache

Small, very fast memory built into the CPU. Stores frequently accessed data to reduce RAM wait time.

Parallel

Multiple tasks running simultaneously — possible when a CPU has multiple cores.

L1/L2

Cache levels. L1 is smallest and fastest (on-chip). L2 is larger and slightly slower.

Multi-threaded

Software written to use multiple threads — takes full advantage of multi-core CPUs.