This isn’t true for everyone. I have witnessed people whose curves are more gradual. Their performance exceeded mine and the gap increased with increasing duration. The principal difference I’d say is task focus. After many hours my attention falters, but I know people whose attention diminishes more gradually.

I could see someone like that using more hours productively. In a sense, someone who is awake 18 hours lives 12% more life than the 16 h me.

I don’t know about that. What I have noticed is people who work long hours often have a distorted perception of their own productivity.