lower clock vs less used cpu power consumption

[theshiznojudge]

my understanding is that transistors are more efficient when they change state slower, and system panel shows my dx clock is 20% - 30% when idle. does that mean that each cycle is 1ns and just doesn't do anything most of the time, or does it dynamically change the clock with demand? im thinking about underclocking it when idle, but i haven't found any benchmarks or explanation as to why it would be more efficient. also, why doesn't everything just have a heat based clock?

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