Question regarding fuel consumption at higher altitude
Firstly, I understand that fuel consumption becomes less at higher altitudes as air is thinner, which results in less interference allowing to fly a longer range.
But isn't thinner air also bringing down the engine performance?
Also, I get that below the optimum altitude, you would burn more fuel as a penalty, but above that, where air is even thinner, why exactly would you burn more fuel.
I think I am missing the main concept.
Explanation please?