The new iPhone 8 / 8Plus / X only comes with normal USB charger. If you want to fast charge your iPhone 8 / 8Plus / X, you need to get a USB-C power adaptor and a USB-C to Lightning cable. And this is a good opportunity to get the genuine Apple accessories, since they are rarely on sales.
- Here are some tests conducted at Gtrusted on various Apple USB-C power adaptors charging iPad Pro 10.5", and 29W power adaptor charges iPad Pro 10.5" fastest. Assuming iPhone 8 / 8 Plus / X use the same charging circuit design, these results may reflect a similar result.
Apple 29W USB-C Power Adapter Can Supply USB Power Delivery Charging to the Apple 10.5-inch iPad Pro
How the Apple iPad Pro Gets Charged by the Apple 61W USB-C Power Adapter over USB Power Delivery
Apple iPad Pro Enjoys Fast Charging with the Apple 87W USB-C Power Adapter over USB Power Delivery