Actually I found that it did charge faster, in a particular way. If the radios were running and the screen was on I found that the higher wattage "power adapters" (Apple's term since the "charger circuit is in the phone"), were able to deliver power to the phone while still maintaining the 5 watts to the battery. When using the cube power adapter that came with the phone, some of it's amperage would be consumed running the phone, so batteries would receive less than the 5 watts the charger could direct to them at maximum, and therefore take longer to charge. If the phone was off or in airplane mode with screen off the iPad power adapter had no speed increase over the cube.
I no longer have an older phone, but perhaps someone with a 5s could run some trials and report back any differences. I do agree that since the 6, the phone can use all of the amps from the iPad power adapter to fill the batteries. I have run a trial using the 2.1 iPad adapter on the 6s Plus and found I could cut in half the charger time over the cube power adapter. I noted that from 0% to 70% the battery would increase by one percent for every minute it was being charged. So like the Geico commercial fifteen minutes will get you 15%. The rate of charge decreased so where in the 70 percent range till at 99% it was trickle charging on and off in maintained mode to protect the battery.
