We put iOS the 12 beta on an iPhone 5S: Here's what happened
https://www.cnet.com/news/ios-12-beta-on-an-old-iphone-5s-what-happened-speed-test/
But one of the most significant changes is performance gains under the hood, especially for older devices like the iPhone 5S.
Apple made some big claims during the WWDC keynote when iOS 12 was first unveiled. Using the example of an iPhone 6 Plus: