It appears the opposite is true, adding salt to water makes it boil faster, but not enough so to make much difference.

I remember my college chemistry professor, L Carroll King, spending most of a lecture on what happens when you boil something. He was legendary at Northwestern for his intro to chemistry lectures. For one lecture, there were two large balloons floating in the room. He told us one contained hydrogen and the other helium and challenged us to suggest ways to tell which was which. After going through a variety of methods, like releasing them to see which went to the ceiling of the large lecture hall faster (hard to tell), he said there was another easier way. He then took a long metal tube, attached it to the gas with a piece of rubber tubing, lit the gas, and touched the flame to the two balloons. One of them went BOOM!, of course. Class dismissed with a bang!

Boiling water is more complicated than it seems, the amount of dissolved minerals impacts the rate at which the water absorbs heat and the boiling point, and the the pressure at the bottom of the pot, which is a combination of the atmospheric pressure and the depth of the water is also a factor. A wide pot will boil faster than a narrower pot with the same amount of water in it, though the size of the pot also affects how much heat is transferred to it from the stove.

This article does a good job of explaining things, though it doesn't really go into why adding salt makes the water seem to boil harder. As I recall, it isn't that it's boiling harder, it's that it has smaller air bubbles due to the presence of the additional minerals, which make it seem more active. (The same thing happens if you add baking soda to a pot of boiling water.)