Wondering why your iDevice is running slowly, well I believe I have an answer for that. iOS 7 and up uses stylesheets much like CSS files for websites. The processor has to generate the UI from the ground up as soon as you open an app and that also applies to notification center, control center, and spotlight search, but you don't have to worry about those because iOS generates the style for those when your booted into iOS.
iOS 6 and older didn't use stylesheets in that fashion. iOS was image based and made the UI much like layers. The navigation bars, background, buttons, etc, were cropped and put into the appropriate spots via a stylesheet pointing out where they are supposed to be.
On a moderately powered device, having to generate the UI from the ground up takes a HUGE chunk out of performance. It's ridiculous, my iPhone 4S almost five years old on iOS 6 can perform just as well as my new iPhone 6S.