I wonder if this is also due to new major iOS versions being released at the same time as new iPhones, and these updates add more intensive features (e.g. apps open in the background) and are designed more with the most modern devices in mind.
I know that my 2011 iPad 2 started feeling much slower after installing iOS 7. (Though luckily I finally tried the "Reset all settings" trick and it made a big difference, which points to a bug.)
I know that my 2011 iPad 2 started feeling much slower after installing iOS 7. (Though luckily I finally tried the "Reset all settings" trick and it made a big difference, which points to a bug.)