My hypothesis: I'd say the hot water will experience faster convection which will continue even when it's cooled down because of inertia. That way the transport of heat to the outside is faster than with the cold water, which will experience slower convection.
that sounds reasonable, but consider this complimentary explanation: temperature itself has inertia, so the (formerly) hot water's temperature momentum is greater as it smashes against the 32F buffer zone, forming crystals instantly
not to mention static vs rolling friction (as in rolling boil)
