This post To Get More Done, Slow Down by Peter Bregman explain clearly enough that we can’t just push harder-n-harder to get more-n-more; there is always a limit. 
Energy (es battery’s charge) is limited, so you need to spent the right amount to the right task in order to get the best.
Optimization, Operative research, ecc always the same things, you know. 
source: To Get More Done, Slow Down
