Iodized salt, also known as table salt, melts faster than rock salt, according to espsciencetime.org. It does so because of multiple reasons, including its surface size and freezing temperature.
Rock salt and iodized salt will both make it more difficult for water to freeze, according to barransclass.com. Iodized salt, however, freezes at a temperature less than 32 degrees, which is the temperature at which water freezes. The difference in temperature between that of air pressure and freezing points for salt water is larger than for the freezing point for water, according to espsciencetime.org, making it quicker to melt than rock salt.