Look, we KNOW Apple can make their built-in apps load in ONE second on an iPhone 6 or iPad Mini 4, they've done it before. Their built-in apps shouldn't change that much to make an iPhone 6 take 4-20 seconds to load them in iOS 11. Period.
Let ignore 3rd party apps for now. Apple is in control of their OS and it's built-in apps, and they get sloppy when it comes to anything more than 2 years old. They should be held accountable. Just making the apps load as fast as before makes the phone more responsive and therefore useable and extends it's life, even if it still takes a long time for Numbers to sort a database or iMovie to compile a video.
An iPhone 6 on iOS 10.3.3 was responsive and useable one day, and toast the next, by virtue of an OS update. They somehow managed to make an iPhone 6 GUI launch apps at a decent pace in iOS 10, TWO years after they launched the phone, but in iOS 11 the GUI on my iPhone 6 runs as slow as my iPhone 4s that's stuck on iOS 9.
That iPhone 4s was great on iOS 8 and likewise iOS 9 killed it. Now, iOS 11 killed my 6, and made my Mini 4 much less fun to use. And 7 8 9 for dinner (joke).