>You make it sound like these are two orthogonal aspects. When rendering is faster, CPU usage obviously goes down.
Only as much as its the rendering, and not the core logic that consumes the CPU.
Degenerative case: a page with a single text entry field, where you enter a number and it calculates e.g. the fibonnaci sequence up to that number or factor primes etc. There's hardly any rendering, but lots of CPU.
You make it sound like these are two orthogonal aspects. When rendering is faster, CPU usage obviously goes down. As does battery impact, since the CPU can go back to a sleep state faster.