As far as I understand the power consumption of your CPU doesn’t scale linearly with the clock rate. In other words, the CPU is more efficient if clocked down.
But I think this isn’t even the deciding factor here.
The CPU is not fully utilized most of the times. That means, if you allow the CPU to go into a higher power state, it will not only be faster, but also burn more energy doing nothing. Even if you have your governor on powersafe, your CPU will not immediately clock down when it is not used. There is always some margin.
Also there are always a bunch of useless background programs which you don’t really need but always use CPU cycles (At least after you open a few browser tabs). These background tasks often eat enough CPU cycles for the governor to decide to clock your CPU high, even it’s not really required.
And what benefit does it bring to you if the invisible animated ad banner on your browser tab 999 runs with 5 or 10 frames per second in the background?
The only really safe and consistent method to prevent this unnecessary drain of energy, is to configure an upper limit for your performance.
It’s true that your computer will be slower, but since most of your CPU time usually goes into stuff you don’t even notice, the resulting loss of time this isn’t nearly as high as the lost battery time which is the alternative.