Performance will decrease, though not to an incredible degree like on, for example, the iPhone 4s on iOS 9 or the 5c on iOS 10 (I have it, it’s absolutely pathetic. It ruined the device).
Battery life though? It will be ruined. iOS 12 is already considerably worse than iOS 9-10. There was a sharp drop on iOS 11-12, and an even sharper drop on iOS 13-14. iOS 15, obviously, worsened it further. I’ll give you some numbers for you to consider: With moderate use and a new battery, you’ll probably get around 2.5 hours on iOS 15, at best. As an example, I have a degraded battery (65% health) on my iPhone 6s on iOS 10, and I get about 6 hours of screen-on time on full LTE. Full Wi-Fi is even better, at around 7.5 hours. iOS 12 would probably get me 3-3.5 hours at best. I reckon I’d struggle to get 2 hours on iOS 15. Raw numbers will be better with a new battery, but don’t expect a lot more than that, like I said.