All I know is that my iPad 2 would run like shit under whatever the last supported version of iOS is on that device.
Excuse me while I rant for a bit.
Ideally, I'd have stuck with iOS 6. That was the last version where the keyboard didn't randomly freeze or close itself, where I could keep two or more tabs open in a browser and even switch out to another app without the need for a page reload, where double tapping the home button instantly and fluidly took me into the task manager. Where animations played out like they ought, mostly fluidly, instead of awkwardly lurching about. I have a task manager like app (lol) that reports available memory and CPU usage, and I have observed a degradation in readily available system resources after most updates. Nothing you can't reclaim, but by default the system becomes increasingly busy with every update and only frees resources on request, which leads to hitching and jerking and rarely even app crashes. Those are things I could verify, no need for synthetic benchmarks.
So yeah, future builds definitely do slow down the older devices.
Also: I know battery life will tank with time. I know Flash memory speeds and capacity/reliability will degrade, especially the crappy flash chips apple used in this early version of their tablets. I know more OS features need more resources. You can't magically squeeze more performance out of existing hardware, and software tweaks only can take you so far. It's natural that an OS and supporting applications/APIs will not run as well on older hardware than the one it is targeted at (i.e. the new hotness that sells for big bucks) solely because it's obvious more man hours will go into optimisation and bug hunting efforts on the new product.
None of that is Apple intentionally slowing down older devices. Or google, while we're at it (Android 4.3/4.4 on my Samsung Galaxy S3 is spectacularly dreadful). Sure, they could waste money on better optimisation and bug hunting on older devices, but modern consumers are way too trigger happy on new purchases and completely disregard repairability or long term viability of product, to put it diplomatically. There's a reason why we have long term branches for operating systems on servers and to a degree even PCs, but not on "disposable" tech like tablets and mobile phones. People are sheep. They pay for the privilege to be fleeced. Welcome to capitalism.
...I mean, I wouldn't mind if they dug out iOS 6, UI and all, and established that as iOS LTB, to be forcibly installed on all Apple hardware older than 3 years or a rebranded iPhone&iPad LE, only supplied with security fixes until the hardware or company melts down. No multitasking. Exceedingly limited cloud stuff. Restricted access to the App Store.
Just a decade of boring never changing stability and security updates, ideally transparently in the background. For me, that'd be heaven. For the modern consumer, it would be an affront to their sensibilities, a return to Nokia and feature phone madness. It's clear we always need to progress and be at the bleeding edge of science in all aspects of life.
*old man yells at cloud.gif*
...or is that HEIC/HEIF nowadays? Pshaw, humbug. Why, back in my days, image and movie file formats kept themselves pure, none of that multimedia shit where every format is basically capable of everything. Cough, wheeze, and no always open mic constantly spying on you on behalf of them newfangled machine overlords. Bought me my stuff at a real shop, talking to a live human being, not like some caveman on their amazons and facebooks and whatnot.