First, you can go back right now (and I think you should).
But most importantly, it is way, WAY too late in the iOS lifecycle for you to realise this. Major iOS updates destroy devices. This has been true since day one. This is true today. This will probably be true forever. Apple has no incentives to make it better. Apple has no incentives to allow downgrading. Because people (as a majority group), if anything, are practically guaranteed to do one thing: update anything and everything as far as it can go.
Today, updating starting from the fourth major version onwards is a massive risk. For many devices, original + 2 seem to be mostly okay. Before, it was original +1. Before that, it was 0. Updating the oldest compatible device? You’re just asking for your device to be garbage.
Things are not as bad. Thing is you merge 2 separate issues, performance and battery life... I think they deserve to be considered separately...
And even battery life itself can be split into screen-on time and stand-by time.
In terms of performance, while back in A7 time updating to iOS 9 slowed down the device considerably (so 2 OS updates), with M1 and newer we have a lot of headroom, enough to update till the end with no performance penalty.
Having said that, one issue that some can consider part of the performance category is reloads, and they keep increasing as the OS and apps take more RAM so even 8GB RAM can start to reload more than some people would like at some point (it's already the case with heavy multitasking).
Apple, probably for battery life reasons vs iPhones, gives same-chipset iPads one more update. A9 iPad Pros; A10 iPad Pros, and now A12 iPad Pros. Both cases thus far are utter garbage. First and second-gen iPad Pros have appalling battery life on their final versions.
Battery life is a different thing. I have never measured screen on time as I don't use my iPads for many hours running, but standby drain has been a big issue with iPadOS. However it's has been bad for a while and has not become much worse (other than in the beta of 26, hopefully because of the beta but we will see). So it's not like by not updating you solve much.
I have been a long-standing proponent of one key rule: never update anything. Keep all devices on the earliest possible major version. When that’s too incompatible, upgrade, and keep the older device for whichever specific function you would like it to perform.
This is something I have mixed feelings about. While it's what I do with my mini 2 (which I use for 32bit apps), I am not sure many people would buy and new iPad and use the old one to do only 1 task (that could be done just as well or better by the new one).
Having said that, not updating (because you can't or because you don't want to) works great on Android, where older versions still do over 90% of things. Take Android 10 from 2019, it still does almost everything. Now take iOS 12 or 13 from that same year, they are becoming unusable become a lot of things no longer work on them...
I am confident my S7/8 on Android 14 will be perfectly fine 10 years for now. Not so sure about my iPad 2018 on iOS 17 (I am still debating whether to upgrade to 18, but it will not see 26).
People don’t do this and update anything. I do blame Apple primarily, but frankly, if today, after nineteen whole years of iOS where the only constant has been “final versions aren’t great”, you keep updating devices to these versions willingly, then you kind of deserve to use garbage, frankly.
Honestly, I think that other than standby time issues, my 16GB RAM M1 will be perfectly fine on its final OS (iPadOS 28), although it will stay on 15 for a couple more years because that's the only way to keep Windows virtualized on it...
