They are! Battery life is extremely reduced when compared to iOS 10 but performance is fine. That said, there’s a massive difference between iOS 12 and 13, and an even greater drop on iOS 15. Even with a new battery, battery life probably won’t be enough even for light users. I tried an iPhone 6s on iOS 13 and I got about 1.5 hours of screen-on time with light use on iOS 13, with about 65% remaining. I would maybe barely get 4 hours. Somebody I know bought an iPhone 6s on iOS 15 (with a new battery!) and only got 2/2.5 hours of screen-on time on iOS 15. I’m a light user, so I might get a little more than that, but it’s still not good enough.We still use iPhone 6S here too as they're awesome phones. Think I'm on iOS 12.
On iOS 12 it is better, though. Not like iOS 10, but it’s not iOS 15-like, luckily. For what it’s worth, I’m getting 7.5 hours of light use on iOS 10 with 63% health.