The better performance of a fit person is partly explained by more muscles (incl. heart), but a significant part of performance boost is the fact that body auto

The better performance of a fit person is partly explained by more muscles (incl. heart), but a significant part of performance boost is the fact that body autoregulates (e.g. by sweating and keeping the temperature perfect for activity, instead of letting body overheat, which results in a performance drop) itself so much better. Raising the heart rate much faster is also part of a better self-regulation of a fit person.