Slow motion doesn’t somehow make defending work better and attacking worse.

I disagree. As an attack comes closer to hitting, the weapon is by the very definition of "closer", closer to the defender than to the attacker - meaning the attacker is increasingly extended, while the defender will pull their defence closer to their own centre of mass. At this point, reactions by the defender require smaller movements, and less acceleration, while the attacker has more momentum in their extended attack, and requires more energy, and more time to change direction.