Suppose you have a cup of hot coffee and some cream at room temperature. If you want the coffee to cool to an appropriate drinking temperature as quickly as possible, should you add the cream right away, or wait a while first?
I would add the cream to the hot coffee right away. By adding a lower temperature liquid, you are transferring a part of the heat to the cream and letting the coffee cool down faster.
On the other hand, if you wait to add the cream, the coffee’s cooling is not going to be much faster. Do correct me if I am wrong.
Why, I don’t know… but intuitively, since the cream is at room temperature (and not cold), it’s not gonna make too much of a difference in lowering the heat of the coffee anyway. This is assuming that the cup is not an insulated mug.