Why % CPU in Activity Monitor Isn’t What You Think
When macOS loads two of those E cores with low QoS threads, it sets them to run at a low frequency to make the most of their energy efficiency. When it loads two E cores with threads that were intended to be run on P cores, as they have a high QoS, it runs the E cluster up to high frequency, so they perform better.
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If full load at maximum frequency is 100% of their capability, then at low QoS they were really running at 27%, not the 100% given by Activity Monitor, and that largely accounts for the difference in time that those threads took to complete their floating point calculations.
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What Activity Monitor actually shows as % CPU or “percentage of CPU capability that’s being used” is what’s better known as active residency of each core, that’s the percentage of processor cycles that aren’t idle, but actively processing threads owned by a given process. But it doesn’t take into account the frequency or clock speed of the core at that time, nor the difference in core throughput between P and E cores.
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