I think this comes down to a level of experience. If you're writing and enterprise app and lean back in your chair and start to think about speeding things up

I think this comes down to a level of experience. If you're writing and enterprise app and lean back in your chair and start to think about speeding things up with loop unrolling and avx instruction sets then you're doing the premature optimization thing. But trying to limit large nested loops is easy fruit that doesn't take much effort to pick.