I am not concerned with the replacement costs -- it's just where I live, getting the battery replacement will likely mean that I have to ship it to another city(Athens), or, maybe, find some place in a different city in the island (Crete) who may be able to do it. Its a hassle and an inconvenience for me.

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.