Nothing transfers heat faster than water. But...when it gets to 212 degrees, air bubbles interfer with the wetting action, as in there is less water to metal contact. So now the bike gets hotter, more air bubbles, less water to metal contact and the cycle begins. Antifreeze (antiboil) prevents that but at a price, namely the ability to transfer heat. One product I use is NPG+ from Evans. It is non-water based and doesn't expand when warmed up like water does, so no spilling coolant past the pressure cap. It can run up to 250 degrees without any problem, first hand experiance.