When boiling water with salt you add the salt to the water after boiling or after the water has reached boiling.
Salt should always be added after the water has reached a rolling boil.
You want to wait to add the salt to the water after the water has reached a rolling boil, to prevent corrosion of the pot and to prevent salt buildup.
Adding the salt to the cold water before boiling will allow the salt to sink and then sit on the bottom, which can cause pitting in stainless steel pots over time.
Boiling also allows the salt to dissolve faster and prevent it from slowing down the boiling process.
Salt crystals have the ability to corrode and pit the bottom of a pot if they sit in one spot while the water heats up.
The boiling and the agitation will prevent this from occurring by keeping the salt moving in the water throughout the pan and boiling process.
Salt also dissolves faster in hot water so you won't have to wait as long.
Also adding salt to cold water can also increase the time it takes to boil the water.
Although the effect on the time for the boiling of the water with the salt in cold water is minimal, adding the salt at a rolling boil is the most efficient method of cooking things, especially for pasta.
If you do add salt to the cold water, you should be sure to stir thoroughly until all the crystals are dissolved before adding the ingredients.