Twilight zone occurence?

I have observed in a system of mine a phenomenon that I’m hesitant to even try and classify.

This system is a older AMD64.

I can run a certain very-high-cpu-using process at nice 0 (normal priority) and the system core will rapidly heat up to 64 degrees C (it used to go considerably hotter and then go into thermal shutdown, but I put it in a case with a obscene number of fans, and even added a couple of extra well-placed CPU fans, and now it stops at 64).

Or, I can run the same process at nice 19 – getting almost the same amount of work done – the process still gets 96% of the CPU, and still performs it’s task at very close to the same rate (maybe 15% slower) – and have the CPU sit at 40 degrees C just as it does when the system is idle.
Can anyone explain this? Does that last 15% really cost that much?

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