Seems like the 5 was an especially good buy. You got a new body and much faster phone. Usually when the body is updated the phone is almost the same speed as its predecessor (3G vs original, 4 vs 3GS, 6 vs 5S)
In the first two cases, the phones only lasted ~2 iOS versions before slowing down significantly (3G slowed down around 4.2.1, 4 slowed down around 6.0)....so I would expect the 6 to slow noticeably on iOS 9 and significantly on iOS 10, unless something changes