This is a well-known legend among users especially those who not fond of the device: the performance of previous generation iPhone would be reduced whenever a new version is announced. According to Futuremark which is known for its benchmark suites for PC and mobile devices, it is huge misconception.
Based on the data that the company collected through its 3DMark suite, iPhone didn’t suffer any huge performance degradation over time contrary to what might have been believed by many users out there. Here is an example for the four years old iPhone 5s:
The same conclusion also holds true with subsequent generation of the device, from the 2014’s iPhone 6 up to last year’s iPhone 7:
If the data set above have clearly busted the myth, how does it still exist among users? Futuremark pointed that it is likely due to perception. For certain apps, they might be optimized only for certain (and usually older) iOS versions.
There are also many instances where developers come out with new features for their apps that require more firepower or designed to take advantage of newer hardware. Naturally, this means they just performed better on newer iPhone model.