Yeah, it kind of sucks. I totally understand though thinking that iOS 12 would perform better than all aspects of iOS 11 after all they hyped it up.
I remember there were a lot of people on the forum with SE’s who hated the battery life drop going from 10.3.3 to iOS 11. Eventually 11.4.1 was decent near the end (which I am currently keeping all my devices on) but the feeling I got was that it still unfortunately was a drop in battery life from 10.3.3. Same went for SE users on iOS 9 that updated to 10 and saw a similar battery life decrease.
I actually hate the fact that Apple doesn’t allow people to downgrade their iOS devices the same way they can with MacBooks. I also hate that they bundle their security updates into iOS updates so that you have to nerf your older generation device with a major update just to have the latest security.
Oh well, enough whining by me
My main point is simply to always expect that any iOS update will decrease your battery life, no matter what snake oil Apple tries to sell it with like they did with iOS 12