iOS 6 feels more responsive because the animations are much quicker. With iOS 7/8 you generally can't tap until the animation finishes (you can't swipe Notification Centre back up until the animation has stopped completely) and because the animations are slightly longer lasting so the OS feels less responsive. The iPhone 5 stutters a little with some animations (bringing up Control Centre on the Springboard for example) though I've never measured whether that slight stutter means the animation also lasts a fraction longer or not so feels less snappy.
I imagine iOS 9 will fix a lot of this since it is focused on bug fixes etc rather than new features.
I imagine iOS 9 will fix a lot of this since it is focused on bug fixes etc rather than new features.