I assume the iOS12 update includes a combination of more aggressive waking and maximizing of performance along with a reverting of what they did previously to old phones when they lowered their performance to maintain the battery life of the degraded battery. If you remember there was quite a bit of scandal about this earlier this year.
My guess is your old iPhone's battery will also last half as much as it did originally within months. Yes, it's a factor of the new changes, but let's not forget it's primarily a factor of Apple using batteries that lose a significant percentage of their capacity after 2 years in the iPhones, and then hiding much of that by slowing your iPhone down. Maybe Apple should put higher quality batteries in their phones - just saying.