iOS 12 is only "fast" if you compare it to the disastrous iOS 11. At some point around iOS 12.2, our iPhone 6 Plus became so slow that we replaced it (with a new iPhone of course). iOS 10 would have been a good LTS release for the iPhone 6.

This trend honestly makes me feel dirty to work in software. If you only use a few core apps, iOS 12 looks and feels just like iOS 7, yet it needs several times the horsepower to work. And unlike on the web, we can't blame it on JavaScript ads. It's just shoddy engineering that aligns with Apple's incentives.