There is not enough
There is Perfect
There is too much
For lack of a better explanation Perfection would create the most bat speed that is under total control. Too much means the hitter loses control, which can create all kinds of problems with balance, tightness, vision, timing, consistent contact, etc. The bat speed does not necessarily increase with the additional effort.
Seeing that perfection is very hard to find, many believe somewhere below your best swing is a good option. It's once a hitter passes that point of perfection, even a little, he is over-swinging.
Probably the best discription of the perfect swing would be to "attack under control".