You can watch this in action by opening Activity Monitor’s CPU History window, where the active residency is shown for each core separately. This is very important in an M1 Mac, because most system processes are confined to the E cores, and almost all apps and related user processes can run on either P or E cores, preferring the P cores when possible.
As far as % CPU goes, though, they’re all the same. They differ in the number and capability of the units within each core, and their maximum frequency. M1 chips contain two different types of core, those designed for Efficiency (E), and those for Performance (P). No distinction is made between E and P cores Put the other way round, if you see % CPU at 40% and your Mac has eight cores, that’s an average active residency of just 5% on each core. Furthermore, when an Intel core uses hyperthreading to boost its performance, scale maximum for % CPU doubles: with eight cores in full hyperthreading, that comes to a total of 1600%. This is the total for all CPU cores, so if your Mac has eight cores, the scale maximum for % CPU is 800%. So if a core were completely idle, active residency would be 0% when no idle cycles were recorded, it would be 100%. As far as I can tell, the figure given is the total of ‘active residency’ for all cores, meaning the percentage of core cycles that aren’t spent in idle. Unfortunately Apple doesn’t appear to define what the % CPU column means, other than stating that it’s “processor capability”. This article tries to explain that what you’re looking at probably isn’t as bad as you think, and how it can be particularly misleading on an M1 Mac. If we see anything over about 20%, we get alarmed and start asking questions, as if % meant out of a hundred. The first place most of us turn to when we’re concerned about the performance of any Mac is Activity Monitor’s CPU tab, where we can see which processes are hogging the cores.
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