Apple focuses on major performance improvements for iOS 12. This is especially important since the iOS 12 update is coming to all iPhones and iPads that ran iOS 11.
Everyone will see performance improvements on the iPhone and iPad but Apple focused n on just how much faster this will make the oldest device running iOS 12. The iPhone 5s will open apps 40% faster, swipe to the camera from the lock screen 70% faster, show the keyboard faster and in general deliver better performance overall.
On newer devices you will see big performance improvements including seeing the sharing options 2x faster than iOS 11 and apps launching twice as fast under load.
Across all of the models, when the iPhone detects usage, the iPhone will scale up processor power to full speed as soon as it can, instead of gradually. This will deliver better overall performance anytime you use your iPhone.
This is one of the biggest reasons to install iOS 12.