Apple A12 chip is a very powerful chip, it is the brains behind the latest iOS devices like the iPhone XS, XS Max, XR along with the new iPads. Ok they run on the A12X chip to be exact.
Now Epic Games is using all that power to bring 60fps frame rate to the iOS version of Fortnite.
This brings the mobile version closer to the console variant of the game, where you have a bit more resolved detail and a few extra effects, but iOS now completely matches the 60fps frame rate of the console version.
Take a look at a short video right below (H/T to Digital Foundry) explaining how Epic Games managed to double the frame rate on Fortnite for iOS.