Why does it look so unnatural?

I agree she has natural limits in running and swimming because of her female physique, but if they want to show she’s faster than him in those sports, at least make it believable.

She looked like she was giving it everything she had in those scenes, while he looked like he was on a casual morning jog or just strolling around his garden. At one point, he even stopped swimming for a bit in the middle.