Betteridge's law of headlines answers this one. Old devices get slower when they're updated to newer OS's because the newer OS's are designed and tested agains

Betteridge's law of headlines answers this one. Old devices get slower when they're updated to newer OS's because the newer OS's are designed and tested against newer, faster phones, and try to take advantage of the expected increase in resources. Old devices that are not updated remain just as fast as ever. And not all OS updates slow down old devices anyway (just most of them). On rare occasion, an OS update will actually speed up old devices.