Imagine you cycle 100km. On one route there is a mountain 100meter high on an other route is not a mountain but a straight way.
Usually it takes longer on the route wiht a mountain. One reason for that is that the time you spend going up the mountain is far more longer than going down. So at average you will go slower.
But how is it possible that with a moutain where the advantage of going downwards is equal to the disadvantage of going upward, at least regarding the objective obstacle.
Perhaps has something to do with the human body, but I can't see how this works.