Properly designed animations are function: they give a sense of place and help in creating a navigation hierarchy instead of having things pop into and out of existence.
> iOS is just littered with gratuitous animations that needlessly slow down the interface
Their gratuitousness is evaluated by you by the "slowdown" they bring. The only slowdown they bring me is when they're not interruptible/concurrent, but that's a bug, like the calculator app's one.
> My two least favorite are the home screen zoom-in effect you get every time you unlock the device
Unless I'm literally trying to race the beam, by the time I move my finger from the home button to a position over the screen the animation has completed. The animation is interruptible and you can tap or swipe right through it.
> and the animated text on typing autocomplete suggestions
How is that slowing you down? You just type <space> which triggers the animation yet merrily continue as the animation proceeds while you are already inputting the next words.
> costly UI gimmicks like 3D touch
To each his own: I use it everyday to peek at things and multitask, it's a real timesaver.