Have you any updates from your own research on your iPhone 6s with iOS12.0?
I have the same user experience as you; but upgrading an iPhone 6s
Plus from 10.3.3 to 12.0.
In terms of performance, everything is great!
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(I cannot believe it after waiting more than a year to update my iOS
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; but true! and happy about it
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), and as you said, maybe even faster on iOS12 than I had on iOS10.3.3, but as trade-off, the battery draining is also much higher. On iOS10.3.3 I was able to run couple of days, and only turning battery save mode at 20% around the afternoon of the second day. Now, I need to turn on Battery Save mode at 40% at the evening of first day if I wanna try to reach my second day without recharging at daily basis (which has no sense to me for a Plus device user).
And as remark, I have exactly the same settings that I used before on iOS10.3.3: all update background apps disabled, BT disabled, Siri disabled, and even new 'Time usage feature' disabled).
I have the feeling that the difference is mainly in two items:
1) WiFi: Looks like all related to that uses more battery. I began to use the toggle of Settings instead of the Control Center, to REALLY fully disable the WiFi when not needed.
2) CPU performance: Seeing how good and faster is now iOS12 compared with iOS10.3.3; I begin to believe that the CPU is used much more at 100% than before by the iOS12 operative system and maybe even throttled at max frequency at longer time periods than before. And THIS is not showed up in the Battery status summaries; because, as said, the battery drains much faster now, but there is not any app remarked having any high usage (nor in front or background).
I would really like to believe that I'm wrong about my above statements and the higher battery drain on iPhone6s/Plus is due a bug; which could be solved in next minor iOS12 updates
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!; but not fully sure about it.
In worse case, we will have faster older iPhones now; but we will need to recharge more often, which also is very annoying and will decrease the battery life, forcing us to buy a new phone in one year (even if performance wise is good enough), or to force to replace the battery by new ones.