First off, the standard USB Lightning cable and charger is the same old fayre, and will charge the iPhone X/8 no slower or faster than chargers that Apple has been shipping with new iPhones for years.
It charges at the usual 4.85V/0.95A (give or take) and will take about 2 hours to charge the iPhone 8, and about 3.5 hours to fully charge your iPhone X or iPhone 8 Plus (about 15 minutes faster than the iPhone 7 and iPhone 7 Plus because the batteries are smaller).