I think it's about the same? I'm not sure really, I would imagine whenever the bottom of the pot gets hot that's when in starts. So you could have it on the highest setting right from the start but that doesn't mean it's going to get hot faster, probably depends on the metal of the pot and how much water you've got in the pot.
The higher the setting the faster the water will boil. Heat is energy. More heat, more energy. The amount of energy it takes to change the temperature of water is what matters. So The more heat (energy) you subject the water to, the faster it will boil. Once the water does boil though you can put a lid on it and lower the heat and it will continue to boil.
It has been my experience that water definitely, without question, boils faster on high heat. We use a gas stove, and on high a pot of about 4 cups of water boils in a few minutes. On medium, it can take half an hour.