The specific heat capacity of water is 4200 J/KG/C. That is, it takes 4200 Joules to raise the temperature of 1Kg of water by 1 degree Celsius.
Conversely, the water must lose 4200 Joules of energy to cause a drop in temperature of 1 degree in 1Kg of water.
Water may seem to cool down much slower than it heats up because the heating up is an active process. I.e., when heating up water, you are putting it on a heat source which gives out a lot of energy in a short space of time. If you were able to create a 'cold' source which extracted energy at a similar rate, the temperature would drop just as quickly.
Answered by: James Kilner, Music Technology, University of York (England)