Another picture of water boiling (I don't know if there's salt in this one...).
So we know that adding salt to water increases its boiling point -- that is, the temperature at which water boils (which it cannot exceed in its liquid form). Presumably, this would mean that raising the boiling point of water with salt would make it take longer to boil.
But this is not so!
Rather, salt water reaches a boil faster than an equal volume of regular water. Oh, sweet counter-intuition. Here's why:
There's this thing called specific heat, which is the amount of energy required to raise a certain amount of a certain substance by a certain temperature. Typically it's one gram of something by one degree Celsius. Water (H20) has a specific heat of 4.184 g/J*ºC, which means that it takes 4.184 Joules (a unit that measures energy) to raise the temperature of one gram of water by one degree Celsius. Water has an unusually high specific heat compared to other common substances.
Dissolved salt (that is, sodium and chlorine ions) has a negligible specific heat compared to water. This means that all the sodium and chloride ions heat up really quickly -- so quickly, in fact, that we can basically ignore the energy that they absorb. Therefore, salt water consisting of 70% water and 30% salt will heat up approximately 30% faster than 100% water. Pretty awesome.*
*If you think stuff like that is awesome, anyway...