Cloning values, collecting iterators into Vecs and then continue the transformation rather than keeping it lazy all the way through. Skipping structs/enums with references.
I thought they meant the case where you go "ugh, I don't want to write a lifetime here" and then change your code, because you have to. If you don't have to, then yes, there's literally no difference.
I believe your parent is implying that if you skip using a lifetime and do something else instead to make it easier, that may be less performant.