When Apple announced the latest generation of iPhones, it emphasized the performance improvements of the A12 Bionic chip, which is also its first chip built on the 7nm architecture. Though those improvements may not be evident in many situations, it seems like at least Fortnite is benefitting from it.
Epic Games today released update version 6.31 of the game, which, in addition to a few new items and some tweaks, also brings support for 60fps on the iPhone Xs, Xs Max, and Xr. This makes Apple's phones the first mobile devices to run at the higher frame rate. Despite being built for gaming, the Nintendo Switch can only run the title at 30 frames per second.
If you're on Android, you may not be completely out of luck, as Epic has given a statement to The Verge saying that it has been making some optimizations to that version of the game. There's no guarantee that you'll be able to get the same smoothness as on the iPhones, but it could happen in the future.
With the number of gaming-oriented devices running on Google's OS, many of which actually support even higher refresh rates, it would seem logical for them to be able to benefit from such an improvement.