The speed at which water heats up has to do with its heat capacity. Heat capacity is the amount of heat energy it requires to heat a gram of material one Kelvin degree. The higher the heat capacity, the more slowly the water will heat, given the same amount of energy added. The heat capacity of freshwater is 4.182 J/(g K) and the heat capacity of saltwater is 3.993 J/(g K). Therefore, saltwater will heat up faster than freshwater.

One thing to keep in mind is that heat capacity is based on the mass of a material and not its volume. That is to say, given the same amount of heat added, the same mass of saltwater will heat faster than freshwater. If you want to compare the same volume, then you need to know the densities. The density of freshwater is 998.21 kg/m^3 and the density of saltwater is 1024.75 kg/m^3. From the heat capacities and densities, we can calculate that given the same mass, saltwater will heat 5% faster than freshwater, but given the same volume it will only heat 2% faster because it is denser. These numbers will be different for different salt concentrations and also for temperature, because heat capacities and densities depend on salt concentration and temperature. All of the numbers I used above are given at 20°C with a salt concentration of 35 g/kg of salt in water.

Also, here is an excellent explanation:
expl anation, please click here
References:
http://www.kayelaby.npl.co.uk/general_physics/2_7 /2_7_9.html
http://www.swri.org/10light/water.htm
http://en.wikipedia.org/wiki/Properties_of_water