I think because they specified Low Power as a separated term so they don't need to include it in performance metrics. They often goes like "high performance, low power circuit.."

But also some people like this author of "High Performance Integrated Circuit Design" includes power dissipation in performance metrics so i think this is kind of relative 

Dear Nam,

I second your point. I think it all depends on what you finally want to obtain at the end of the day. Suppose, you need to design a low power IC. So for such design, reduction in power dissipation should be important aspect rather than speed and all. Therefore, at the end of the day if my chip yields low power dissipation, I can say I've achieved good performance, right? 

Yes, I think you're right. It all depends on what you want to prioritize

Almost all systems designed today, from high-performance servers to wireless sensors, are becoming energy constrained. Power is also wasted indirectly when we waste performance. Higher performance requirements necessitate higher-energy operations, so removing performance waste reduces energy per operation. Using multiple simpler units rather than a single aggressive one, therefore, saves energy when processing parallel tasks. Continually increasing performance in this new era requires lower energy per operation, because the product of operations per second (performance) and energy per operation is power, which is constrained. Power gating have become common techniques to reduce direct energy waste in unused circuits.

http://www1.cs.columbia.edu/~cs4824/handouts/shacham10.pdf