[G3 General] engmia:
You are on the right path to the truth. There are three main problems here (outside of the device): myths around the community, unconfirmed information and your expectations as a user.

1) Myths around the community: Do not assume that just because you are in a technical community (which implies people craving to be accurate) that there aren't a plethora of myths going around. Especially with the mass increase of new users of the platform.

For example: Reducing screen resolution from 4K to 1080p is a myth. As in battery increase is so negligible it isn't even worth mentioning. This has been side many times around the forum by many people (myself included) but people seem to love doing weird stuff to their phone, without actually verifying the technical data and assuming they work based on their expectations (placebo effect).

Companies like LG do in-house testing. They have done the benchmarks on 4K vs 1080p long time before the phone got into any user's hand and made their decision based on that. Assuming that the company didn't do that is insulting both ways: for one insulting the company that you assume it hasn't done this basic procedures, and second, insulting yourself as a customer, supporting a tech company that releases products without any testing and etc.

2) Unconfirmed information: This is very similar to one, but has to do with the build up that leads to number 3. People sometimes post all kinds of crazy numbers and claims around the forum. For example, a user on the previous page, boasts that 3h of screen time is horrible, yet he proceeds to say that he gets 4 hours of screen on time with lowered brightness.

Well of course, lowering your brightness is one great (real) way of reducing your power usage. However it renders the phone unusable bright light and direct sunlight.

Posts like "I just installed this kernel and noticed 100% increase in my battery life!" or "I greenified this and got 10 hours screen on time" usually turn out to be utter bul****. A kernel tweak cannot give much of a boost in the first place, if there wasn't an inheritent problem in the kernel in the first place. Kernel tweaks are useless for most users that aren't developers and don't know what they are doing.

Increases in battery life usually come at crippling costs, like disabling radios, disabling background applications and etc. E.g. you are just turning features of the phone off.

3) Your expectations: you want a phone that lasts you 7 or 8 hours screen on time and has the functionality of a smart phone? There isn't one on the market today. Look at number 2 above.

Phone, cars, among other products, are currently limited by battery technology. All modern electronics uses LiPo batteries in them. Do not expect a large increase in capacity before there is a new battery revolution/technology around. That's why you see these new phones that charge in 30 minutes. Because the industry obviously recognises the need for better batteries in our devices, but currently there is no technology to allow it. So instead, they focused on what they currently can do -- reduce charging time.

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3 hours of screen on time is normal for the phone. At least you now have two swappable batteries!

The above text comes from knowledge I have acquired over time, and my own experiences with obsessing over battery life, and trying numerous tweaks and kernels to increase battery life capacity. Note that those tweaks were done on my previous Note 3 phone -- so the complaint is pretty much universal.