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.
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.
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.
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.