So... you know this when the beta has yet to be released? Would you like some facts to go with your opinions?
https://arstechnica.com/gadgets/201...e-6-plus-and-ipad-mini-2-its-actually-faster/
Odd that iOS 12 is faster than iOS 10... But still not enough. I have tried an iPhone 6s on iOS 9 and an iPod Touch 5G running iOS 6. Every single app on both, barring one exception (photos, on the iPod Touch 5G, at 1.2 seconds IIRC), were under 0.9 seconds, with the majority being under 0.8 seconds. Do you want an app launch cripple example? The iPod Touch 5G on iOS 9. Native apps took three times at much, with the majority being at or around 2.3 seconds. Keyboard lag was everywhere, battery life was awful.
Now, app launch might be fine, but how's third party app performance, and, most importantly, how's battery life? I'd say battery life is almost slightly above half; and battery health is largely not an issue unless severely degraded.
Besides... I did say iOS 12 was the exception on performance. All the rest were awful, if compared 2+ iOS versions behind (generally, the first version is fine. For example, the iPhone 6s and the 9.7 iPad Pro perform on iOS 10 just like they did on iOS 9, battery life included. Go two versions up, though... And it's a different story).
If we take that into account, then barring the only exception on iOS's history, it is a fact that iOS updates are a detriment to performance and battery life.