[wallacoloo]

The power difference may actually be immeasurable, or even an improvement on some systems. Switching from one power state into a lower-power state may actually involve one-time energy payments, which are eventually negated by the power savings as time progresses. In an extreme scenario, a core might power-down its cache to reduce leakage current, which involves sending all of its pending writes onto some bus, which is costly, and then it'll be reading a lot of data back in from elsewhere once it is powered back on as it refills the cache.

In the worst case, you might spend more time and energy switching between power states than you actually spend in a lower power state.

But indeed, it does seem counter-intuitive even with that, as there are often power mode changes available that would pay off in just a few microseconds. It sounds to me like x86 may suffer from a limited IRQ system - there are other systems out there in which IRQ overhead is < 10 cycles.