Power suply Watts and charging time - Apple iphone
Apple chargers have distinctive "charger signatures", in a form of different voltage combination on D+ and D- lines, see for example this answer, and many-many similar answers. That's how an Apple device "knows" what is the power capability of a particular charger, and takes more current (by changing input parameters of their internal battery charger) and therefore charges faster from a 12W charger than from a 5W charger.