I think the analogy here is backwards. The better question is "how much would you prioritize a car that used only 0.05 liters per 100km over one that used 0.5? What about one that used only 0.005L?". I'd say that at that point, other factors like comfort, performance, base price, etc. become (relatively) much more important.
If basic computer operations like loading a webpage took minutes rather than seconds, I think there would be more general interest in improving performance. For now though, most users are happy-enough with the performance of most software, and other factors like aesthetics, ease-of-use, etc. are the main differentiators (admittedly feature bloat, ads, tracking, etc. are also a problem, but I think they're mostly orthogonal to under-the-hood performance).
These days, I think most users will lose more time and be more frustrated by poor UI design, accidental inputs, etc. than any performance characteristics of the software they use. Hence the complexity/performance overhead of using technologies that allow software to be easily iterated and expanded are justified, to my mind (though we should be mindful of technology that claims to improve our agility but really only adds complexity).