What is a more efficient thinking process? I'm a bit of a perfectionist and strive to make the best move every game. As a result I think I expend too much brain power throughout the game and get burned out or overwhelmed by the possibilites, for example in the opening I would try consider all my opponents responses of which there are many in the opening, sometimes I feel habits such as these are unnecessary and are just a waste of energy. What is the most correct way to expend your mental energy in a chess game?
When you learn an opening you learn sequences of moves and the ideas behind those moves. If your opponent follows "book" then you don't have too much to think about. Probably just "Which variation am I going to play today? Am I feeling like a quiet strategic game or a wild tactical game?"

If your opponent plays a move you are not familiar with then there are two questions and two situations to cover.

1) You are playing a sharp, tactical opening where there are lots of threats and he didn't counter your threat and isn't counter threatening anything. In that case carry out your threat!

2) You are playing a quieter opening in which you are both just developing your pieces. In that case, after checking for threats, ask yourself if his move stops you developing the way you wanted. If it doesn't then carry on. If it does then work out how you now need to develop the rest of your pieces. 

In the middlegame it is a bit different. When it is your turn you do need to calculate what you evaluate are your opponent's best replies. 

When it is your opponent's turn it depends how many good looking possibilities he has. If it looks like just one or two forced or "best" replies then stay in your seat and calculate those. If it is a complicated position with lots of equally likely looking possibilities then either take a step back and look at the position from a long term strategical point of view or save your energy, go get a cup of coffee or glass of water or relax looking at other games whilst keeping half an eye on whether your opponent has moved yet.