You can get a replacement battery for your phone for €25. If you don’t want to do the replacement yourself, any 3rd party repair shop will replace it in 15 minutes. You can also go to an authorized service provider, but that will cost €100 and take much more time.
And, as I said, you are not forced to upgrade. A battery replacement is cheap and quick and will solve all your problems and your phone will feel like new.
I also have the iPhone 6S, and I've also been having trouble with the battery. I've had to replace it twice. (First time Apple replaced the whole phone because of a manufacturing defect, second time I went to a 3rd party repair shop)
Sure, it would be nice if Apple products lasted forever without service. Unfortunately they don't. But servicing iPhones has become extremely cheap and extremely quick -- as long as you stay away from authorised service providers.
What is odd is that is seems to be related to rendering; sometimes when I scroll on either direction on Tweetbot or on Safari, the iPhone will stall for maybe 500ms and the battery will drop by 20% or so instantly. Other times, I will app-switch to Slack and it will do the same. Reading content in black background/white text on the other hand, doesn't seem to be draining the battery. So I think its safe to assume this is an upgrade related problem. Also, I have disabled spotlight indexing and pretty much everything else I could, and it's also been quite some time since the upgrade for any upgrade tasks to be running to completion still.
I’m pretty sure that there is no way to fix your phone with software. It’s a hardware problem. Just replace the battery, and it’ll work fine again.
I don’t know where you live in Crete, but typing “iphone repair cityname” in google should quickly get you to a place that can replace your battery in 15min.
What are you doing on your phone for that type of battery life? My phone (iPhone 6 Plus) has definitely been slower, but my battery life seems to be the same post upgrade. My phone is mostly idle throughout the day.
There were definite performance and battery life issues with 11.0 most of which were fixed with 11.1 and improvements are coming as well with 11.2. For me personally it's on par with iOS 10 for battery life.