Because the salt particles would essentially get in the way of the water molecules forming little bubbles, as the energy increases during a boil in an attempt to turn into vapor to make their way through the pot, adding salt would actually impede progress. On the other hand, salted water does have a lower boiling point than fresh water. A large amount of salt would have to be added to a boiling pot of water for that to make a difference, however, which wouldn’t be appetizing for your palate and could ruin a meal.