Sure and I have full right to ask for proof since that’s what you “accuse” Apple of doing. I don’t doubt planned obsolescence exists; it has to to make consumer goods affordable. It just doesn’t exist in the nefarious way you allege. All of that stuff eventually comes out and I’m waiting for the inevitable breaking story that won’t happen.
[RANT ON] If Apple isn't engaging in planned obsolescence then that means that Apple just doesn't give AF about optimizing their newer iOS versions to keep the older devices responding as snappy as they used to. That's just as bad.
It doesn't need to do anything faster than before, nor do I expect it to be as fast as newer models with faster processors, but it certainly SHOULD NOT BE DOING THINGS SLOWER THAN BEFORE!!! Yes, I'm yelling in all caps.
Apple has not introduced a new iPhone that didn't have a fast response time between telling it to do something and it starting to do that something. Show me a 3 year old iPhone or iPad that's on a newer iOS that responds like it did when it was new.
Benchmarks not changing that much doesn't matter when every-day actions are slow as molasses now.
IT JUST TOOK 22 SECONDS TO OPEN SAFARI AND LOAD THE PREVIOUSLY OPEN MACRUMORS WEBSITE ON MY 128GB iPHONE 6!
IT JUST TOOK 9 SECONDS TO LOAD MUSIC, 8 SECONDS TO LOAD PHOTOS, 7 SECONDS TO LOAD SETTINGS, AND 4 SECONDS TO LOAD CALENDAR, REMINDERS and NOTES!
SERIOUSLY?!?! Don't tell me it was that slow before iOS 11. You'll embarrass yourself.
Now, if the apps are already loaded they launch much faster, but with the limited RAM once I play a few games on the 6 then the standard apps start to take a long time to load again. Once inside an app, performance doesn't seem that bad, but when turning on the device and launching apps the 6 is now slow AF.
We have two iPad Mini 4 here - mine is on iOS 11.1.2 and my wife's is still on iOS 10.3.3 and everything objectively launches and opens much quicker on her's with the old iOS, just like mine did before the upgrade. Whether launching an app or getting the keyboard to pop up, mine feels more sluggish in comparison to hers (and vs how it was before).
My iPad Air 2 on iOS 11.1.2 is no longer slightly faster than an iPad Mini 4 - it's halfway between my Mini 4 on iOS 11 and hers on 10.3.3. Even then, at least it's not as bad as my iPhone 6, and the Mini is still useable.
Typically, I'm finding that the older the device, the worse the issue - my 1 year old iPhone 7+ is only very slightly slower than my son's 7+ which he still has on iOS 10.3.x, so I hardly notice the difference and I'm happy with iOS 11 on the 7+. I don't have my 6s+ to compare speeds anymore, but my daughter's 5SE is still fairly snappy on iOS 11.
But my 6 is useless now, about as slow as my old iPhone 4s on iOS 9.3.x, and worse than my Mini 4 and Air 2 of the same age. Fortunately I've been able to leave my other daughter's iPhone 6 and our spare 5s on 10.3.3, as well as my wife's iPad Mini 4. My son's 7+ is on 10.3.1 which ticks me off that he blew it and can't get 10.3.3 now. [/RANT OFF]
I agree not all devices slow down, much. I still have NO idea why my old 5s is still pretty responsive on iOS 10.3.3 when it shipped with a much older version, but it's probably a fluke just like your older device running faster on iOS 11. My 5s on 10.3.3 runs faster than my iPhone 6 or Mini 4 on iOS 11, and if the 5s was my only device I'd probably be an "Planned Obsolescence Denier" too.