While it does seem like adding potatoes to already boiling water would shorten the cooking time, adding them to a pan of cold water will allow them to cook more evenly. Because they are so dense, it give the inside more time to come up to temperature with the outside and will result in a potato that is cooked evenly all the way through. I've been told a good rule of thumb is that if it grows below the ground start in cold water and if it grows above ground start in boiling water and this has always worked well for me.