Not a scientific test but I just got home and plugged in my iPhoneX into the apple brand 12W usb to Lightning. Went from 21W to 19-16Watts and charging. Charged for about 15mins got 12% charge. Did the same with the 29W adapter and usb-c to lightning. Same amount of time. About the same 12% in 15mins. Granted if I used the usb charger that would take forever. But I would’ve thought it would still be faster than the “old” fast charge of the 12watt adapter.