http://wccftech.com/iphone-8-iphone-8-plus-iphone-x-support-fast-charging-using-apples-usb-c-power-adapter/

Apart from the addition of wireless charging, the iPhone 8, iPhone 8 Plus and iPhone X support fast charging using Apple’s USB-C Power Adapter.

Up until now, the iPhone was not exactly ‘fast charging’ capable. The best the smartphone could do was up to 12 watts of power draw, while the competition was soaring north of 18 watts, thanks to technologies like Quick Charge 3.0 or the USB Power Delivery standard. Now you’ll be pleased to learn that Apple too has joined the fast charging party, with the iPhone 8, iPhone 8 Plus and iPhone X supporting high amounts of power draw for charging up using Apple’s official USB-C Power Adapter. This bit of information comes straight from Apple’s very own website.