With respect, I don't think this comment is dumb. Apple devices, and particularly the mobile ones, represent a careful calibration of hardware and software to work the best. For example, iOS 16 is designed to work best with the iPhone 14 family. Apple engineers spent a considerable amount of time and effort tooling and optimizing iOS 16 to make the iPhone 14 shine its best. When iOS 17 comes out, they will make the same effort for the iPhone 15 family, and somewhat less of an effort made for the iPhone 14 family. As the difference between OS and hardware gets larger, it gets harder and harder to justify the effort to tweak the OS to make older hardware work as well (especially has the newer hardware gets more speed, different architecture, newer features, etc.). Eventually it's not even worth it to make the newest OS compatible with older hardware. But part of that trade off is that your older device will run slower on the new OS, both because the new OS has more hardware-expensive features AND because the OS isn't AS finely tuned for the older hardware.
(Fwiw I don't think the tuning makes THAT big of a difference, but its probably fair to say that it is less good than the day you bought it.)