Hwinfo64 can tell you the percentage of time the cpu speeds in the idle state.

Doesn’t idle mean something more like C:\> than multitasking billions of instructions per second?

We're talking idling from the perspective of an actual user at home, so there will obviously be a web browser and some background tasks running.

Idling without anything running, even an OS, is interesting for metrics, but not something realistic for any user.

I agree that we should be comparing real scenarios, but I also notice that my desktop Linux systems tend to idle at <=1% CPU use if I don't have a browser open, so there's probably some genuine room to consider what counts as idle even with an OS running.

Reporting the clock frequency of a core requires running a bit of code on it which necessarily means the core is in an active state while you take the reading, so you will always read something like "2.5 GHz" even if the core does not execute anywhere near 2.5 billion cycles while you do that. Some tools can report metrics like "average effective clock" which count time spent in inactive power states as zero clock.