As it mentioned, the thermal mass of the pipe. My first home in Florida, built 1989 had cooper under the slab. I do not know if any insulation was around the pipe but I doubted it. The kitchen sink was at the opposite end of the home from the garage water heater. Not only did it take time but the copper laying in damp sandy soil really sucked the heat out of it even after a few minutes. My current home its CPVC under the slab and it also sits in sandy soil. Since CPVC doesn't conduct heat as readily as copper, the cool down time is longer. It also doesn't chart the ambient temperature where the pipe sits. That would be some good information to have. Probably someone has already performed it.
