it takes time to bring CPUs out of sleep states, and during that time the work that is waiting for them may not be able to get done.
it is not always less power to run at full speed and then sleep, that is frequently the case, but it depends on the ability to move in and out of sleep, along with the amount of power saved.
In this case, we are talking about the options when you have multiple cores, some sharing components, and have less work than it takes to max out all the cores.
putting all the work on one core and powering off the other cores may save power, but it could make the work take longer (but not enough longer to use more power than the other cores would consume if they were not powered down). for some people having the work take slightly longer won't matter, for others it will.
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