You pretty much nailed it here. It's not speed proper. It's the perception of speed. What the iPhone mastered was the transition starting right away. If you have no transition, the time to start, say, the mail app, will appear long, but since you started the icon blowing up to cover the screen right after your finger press was detected (by your brain) the delay feels shorter because you see something is happening. It's merely cosmetic - the app is still starting during the animation - but , to the user, the animation is part of the process.
Err, seems like you got it the wrong way around. That was initially the reason, but these days the animation ends up taking much longer than the actual processing. GP's workaround changes the animation durations to be somewhat closer to the actual time required.
But even that's overkill for modern phones. I just tried turning off animations entirely, and things still feel pretty much instant, despite the phone being a few years old at this point.
