The apps haven’t loaded instantly. Maybe your definition of instant is different than mine. For me instant could be 1-2 seconds or less. Also depends from app to app. Settings must load in 0.4 seconds or less. Keyboard should have no input lag. The fact of the matter is Apple’s updates ruined the phone. It was working fine on iOS 8.The apps load instantly, if the phones’ hardware is the issue you’re just realizing what some on the forum has been saying for years.
By my definition of instant no iPhone below the 6s loads apps instantly.