I hear this conspiracy theory so often from non-technical people. I think what's going on is that obviously this isn't being done intentionally, but rather each

I hear this conspiracy theory so often from non-technical people. I think what's going on is that obviously this isn't being done intentionally, but rather each new release is mainly written for and performance tested on the newest Apple devices (not to mention obvious hardware differences like CPU instructions/speed/cache size/memory). Apple might test that everything works, but they're not on these older devices 24/7 that they'll notice slight degradations. Also, devices are not uniform. On some devices the flash memory has been written to countless times and degraded. Other people are running tons of crapware or have the storage full to the brim. Finally there's always going to be some psychological suggestion going on: if I tell you to look for a certain thing, you'll be more likely to notice it, real or not.