In all honesty, the iPhone 5 has aged better than the 3G, 3GS, 4 and possibly 4S. I had the 3GS and 4 with the 2nd last and last version of iOS they ran, and the 5 feels more modern. The 3GS with iOS 6 does run very smooth though compared to some modern software.
I do believe that post iOS 7 (iOS 8 and 9 in particular) that even on new hardware, iOS does not run as well graphically. I never saw stutter or anything with say the iPhone 5 on iOS 6 or the iPad 4 on iOS 6. Even iOS 6 ran pretty much perfectly on my iPad 2. However, iOS 9 can be made to stutter fairly easily on the iPad Pro or in the iPad Air 2/iPad Mini 4.
Maybe iOS 10 will fix up metal.