Glossary
Performance and efficiency cores
Apple silicon's two kinds of CPU core: fast performance cores and low-power efficiency cores.
What it is
Every Apple silicon chip has performance cores for demanding work and efficiency cores that use far less power. macOS decides where each thread runs, and tends to put background work on the efficiency cores.
That's why a Mac can show busy efficiency cores while it feels perfectly responsive: the background work is staying out of the way.
Why it matters
When the performance cores are saturated for minutes at a time, foreground apps slow down and the Mac warms up. Busy efficiency cores alone rarely matter.
In Activity Monitor
Window → CPU History shows a graph for each core.
In iKnowMyMac
The CPU tab header shows how many performance and efficiency cores your Mac has, alongside total usage and load average.
sysctl hw.perflevel0.logicalcpu hw.perflevel1.logicalcpuPerformance cores (perflevel0) and efficiency cores (perflevel1).
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