The iPhone 8 supports two different fast charging standards.
The primary is USB Power Delivery. That requires using an 18W (or better) USB-C PD charger. And a USB-C to Lightning cable (MFi certified guarantees power pass through). It can charge the iPhone up to 15W, though usually it is lower when measured.
The second is Apple 2.4A. This is an old standard created by Apple when they released the iPad and was included with the iPhone 4. It has been present on iPhones since. It requires an Apple 2.4A compatible USB-A charger and a regular Lighting cable (USB-A to Lightning). It can charge the iPhone up to 10.5W, though again it is usually less.
USB PD is faster when the iPhone's battery is lower. As it charges Apple 2.4A catches up. By 70% charge all fast charging stops.
I have more details on fast charging newer iPhones here.