You need to boil water for tea. Which heats up slower, the pot or the water?
In short, the water heats up slower than the pot used to heat the water. The heating element (either fire or a burner) has direct contact with the container used to hold and heat the water. Based upon this direct contact the pot would heat up faster. Given that the pot is a solid, and the water is a liquid, the water is slower to heat up.
Another way to look at this is that the exterior of something, in this case the pot, heats up faster than the inside. The heat has to transfer from the pot to the water in order to heat the water. Keep in mind though that different types of metal heat at different temperatures. Therefore, it may take longer for some pots to heat over others. Regardless, the water will still be the last hing to become hot.