in a boxing punch, is a short time of impact giving more powerful consequence than a long time of impact, for a *same* speed at impact?

in a boxing punch, is a short time of impact to the opponent giving more powerful consequence than a long time of impact, for a same speed at impact in the two variantes, or is it giving exactly same impact ?

From physics law, I would guess that it gives exactly the same : energy is $0.5 m v^2$ so the duration of impact does not enter in the topic, and the amount of momentum transfer $mv$, the duration does not enter in the topic as well.

If so, why do boxers say that they should have the smaller time of impact ? The only reason I would find is "in order to give the next punch".