It didn't do with iOS 5 or 6 or 7.1.2. What did they change?
iOS 7.1.2 definitely stuttered if you used iPads at all.. Rotating keyboard, rotating app store, rotating anything with a dim overlay, control center over keyboard, control center over lockscreen, sending messages, notification center over keyboard, notification center over lockscreen, blurring for Siri, rotating power down screen (7.1+), using multitasking gestures at all with a keyboard onscreen or when in certain situations with overlays, etc. Spotlight pull down lagged on EVERY DEVICE EVERY TIME. iOS 8 introduced Safari keyboard stutter when tapping URL bar, quick reply has always been laggy (keyboard popping up especially). iOS 8 worked to resolve the Spotlight pulldown lag by simply stopping the icons from going down past the point where they would chop.. But a blurring effect was added. This caused stutter in the pull down and in the keyboard popping-up animation. Rotating spotlight is now a mess because of the blurred home screen AND keyboard.
iPhone 5 and up were pretty dang close to perfection, but 4S and below has always been a disaster since 7.1.2. Control Center and Notification Center stuttered tremendously on 4S and iPod 5 at least. Pretty much anything that made iPads stutter above made iPhone 4S or iPod 5's stutter as well. Also, opening new tabs in Safari was awful on these devices. still is today.
iOS 7.1.2 was not perfect like you say. iOS 6 was definitely MUCH more fluid on every A5+ device, at least. 3GS and iPod 4 struggled when performing animations that hadn't occurred in awhile. Most notably, exiting an application after awhile, and I *think* I recall my iPod 5 stuttering when closing an especially heavy application but other than that it was always incredibly smooth and quick, dare I say perfect? But anyway, on the older devices I mentioned, you could close an app and have it be an awful mess, but open it up and immediately close it again and it would be smooth. This same thing happens on my iPad mini 2, pretty much any device with low RAM, as it is a RAM related issue. This is because animations are cached in memory I believe until it has to be cleared as a result of another more important task needing more memory.