The slow heating and cooling of liquid water are caused by its high specific heat capacity.

Liquid water heats up and cools down slowly because of its high specific heat capacity. Specific heat capacity is the amount of heat energy required to raise the temperature of one gram of a substance by one degree Celsius (or one Kelvin). For water, this value is notably high, about 4.18 joules per gram per degree Celsius. This means that it takes a lot of energy to raise the temperature of water, as well as to lower it.

The primary reason behind water's high specific heat capacity is the strong hydrogen bonding between water molecules. Each water molecule is capable of forming up to four hydrogen bonds with its neighbors—the two hydrogen atoms can each form a hydrogen bond with oxygen atoms of other water molecules, and the oxygen can form two more with hydrogens from other water molecules. These hydrogen bonds are quite strong and thus require a substantial amount of energy to break. As a result, when you heat water, much of the energy goes into breaking these hydrogen bonds rather than increasing the kinetic energy (which raises the temperature) of the water molecules. Similarly, when water cools, it releases a significant amount of energy when these hydrogen bonds are formed. This process of breaking and forming hydrogen bonds eats up a lot of energy, making water's temperature rise and fall more slowly than that of many other substances.