I did a test on two laptops one with Win7 and an i5 xxxxM processor, and one with 10 and an xxxxU processor. (U is lower power than M. Nowadays there is also a Y series that is lower power than U).
This pic shows a really good result from the Windows 10 i7 xxxxU machine.
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/lM3FC.png)
In the sense that it shows a direct relationship between on the one hand, the figure for max processor, and on the other, both CPU load and CPU frequency.
Best result for lowest CPU load and lowest frequency, is no more than 30% written in for max processor percentage, in power options. Less than 30% doesn't seem to affect CPU load percentage to any greater extent. It looks like 30% is as low as it lets you go.
The result on the Windows 7 i5 xxxxM machine was a bit different, not quite as good.
On the windows 7 machine with the i5 xxxxM processor, it wasn't reducing the load, but was reducing frequency. Frequency could go down to 30%. But load remained at 100%.
more info here https://pastebin.com/raw/at9j45Fr
but I will improve this answer when i've done more tests.