I have iPhone 8, generally iOS 15 runs fine. However, occasional lag and app crashes happens from time to time. Performance is similar with iOS 14 and battery life seems fine. But due to iPhone 8's small battery, I have to charge twice a day (brand new battery, battery health at 100%).
I don't think there are enough compelling reason to go iOS 15 giving iOS 14.8 is stable enough. Lots of new features, like Live Text, new Maps aren't available to iPhone 8 anyway.
There’s nothing compelling enough in iOS 15 to make the jump from 14 on an 8. Stay on iOS 14 until August- then upgrade just before the mess called iOS 16 drops.
In my opinion, the only time to install a newer version of iOS is in August when it’s done. Then you’ll only need to install it once when it’s “Finished” than suffer through the yearly time wasters of weekly updates all year fixing one bug after the next.
Launch day versions of iOS is for the birds- it’s never polished - it’s semi functional beta software.
Upgraded to iOS 15 on day one and didn’t face any issues. Performance and battery life was largely the same as compared to iOS 14. Just that there really wasn’t any new features that struck my fancy. Didn’t bother with a fresh install either.