Perhaps it's not the speed of the sperm or its potency, but the LOCATION and DEPTH of the DEPOSIT????
Perhaps?
Plus, it is now established that when a woman has an orgasm AFTER her lover cums inside of her, her cervix (the opening to the womb) actually drops down and dips into and sucks in the pool of sperm that is deposited inside of her. That's just how nature works....nature rewards the guy who gives her the BIG O after he cums inside of her by helping her get pregnant with his sperm.