I used to write a lot of performance-critical Java (oxymoron I know) for wearables, and one time I got a code reviewer who only did server-side Java, and the differences in our philosophies were staggering.

He wanted me to convert all my code to functional style, using optionals and streams instead of simple null checks and array iterations. When explained that those things are slower and take more memory it was like I was speaking an alien language. He never even had to consider that code would be running on a system with limited RAM and CPU cycles, didn't even understand how that was possible.