Did the use of Supercooled LOX made the difference?
It was the other way round. When they switched to subcooled LOX, keeping the LOX subcooled became a problem: the longer the LOX spends in the rocket, the more time it has to absorb heat from the surrounding air. The LOX then has to be cooled down by bubbling cold helium through it, which makes the fueling process more complicated.
Filling the tank rapidly is something you normally want to avoid, because it increases the stress on the tank (because it has to cool down rapidly from ambient temperature to the temperature of the LOX). When SpaceX switched to subcooling, they may have had no other choice than to increase filling speed due to the above.