Why does my hr slow down so quickly after exercise?

That's a good thing! A heart that is used to vigorous regular exercise is supposed to slow down quickly after exercise. In fact, having your heart rate going too fast for too long after exercise is sometimes a sign of silent heart disease, even if the person is a well-trained athlete with a slow resting heart rate. Now if you don't exercise regularly & your pulse is under 60bpm, talk to your doctor. That's a problem.