Matthew Garrett on the race to idle

You still have to allow for the fact that the rest of the system (disk drive, display) is
still drawing power.  It's like speeding up a small section of code by a factor of ten -- if
that code only runs 10% of the time, you have only speeded up the entire program by 9% (isn't
this Amdahl's law?).  Factoring in just the CPU power doesn't answer the question properly.
You may end up using more power and drawing down your battery even further.

In fact, TFA said this.