Because many pasta and potato recipes call for salt to be added to water before boiling, many people erroneously think that this is intended to make the water boil faster.
At home, even with the water and salt measured into exactly equal proportion, it would be difficult to tell for sure which of two pots boiled first. There is also the matter of environmental discrepancies, such as uneven stove burners, pots with bottoms of different thicknesses.
The addition of salt to water actually raises its boiling temperature. For every ounce of salt to gallon of boiling water, the boiling temperature of the water is raised about -8.33 degrees Celsius. In fact, any non-volatile substance raises the boiling point of water—that is why you add ethylene glycol (antifreeze) to car engines in the winter.
The reason we add salt to boiling water for pasta and potatoes is mainly to add flavour. Technically, if enough salt was added, it could speed up the cooking time of the pasta. However, that would make the pasta too salty to eat.