Don't do it. A friend had a 6+ on iOS 11 and battery life was lower - yes, lower - than my 6s on iOS 9. (A plus should have anywhere between 1.5 to 3 more hours than a regular. Lower? It's the equivalent of my iPhone getting 3 hours of usage or so).
Performance was poor. Really poor. He complained of apps crashing, well, of course he complained about battery life as well.
If you have held out this far, then you don't have issues - like me - with running older versions of iOS.
There is a huge chance - not to say it's a sure-fire bet - that battery life will severely decrease. Performance differences between 9 and 12? Not sure, I can't answer that. But if it works fine, then don't fix it. Mainly because there's no way back, and chances of something going wrong are high, even more so if we consider iOS 12's track record with battery life. Even if performance is good, I doubt it will be better than iOS 9, and battery life is almost certainly going to severely decrease.
PS: Still happily running iOS 9 on my iPhone 6s.