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.
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.