Since it's a dual cell battery it splits the wattage across two of them.
3 easy steps to not murder your battery with fast charging:
1. Don't charge overnight unless you have to
2. Plug it in when you wake up. Go brush, make coffee. Viola you're phone is at 85%
3. Try not to charge to 100% unless you need to.
Also I've got a free battery replacements coz of some care plan. To add to that batteries are cheap as hell and even if I'd have to replace it in a yr the convenience and time saved are worth it to me.
It is the current that charges a battery and the higher voltage is needed to be able to constantly push the same high level of current. Once it reaches peak operating voltage, the current has to be reduced as it causes a lot of heat generation. This heat is what deteriorates the battery and the fast chargers that maintain these higher current levels at high voltages for fast charging are the ones that kill the battery quickly. To prevent high voltages, some of these large charging bricks use Buck converters to step down the voltage but that comes at the loss of efficiency which is why you have these bricks heating up a lot. There is no beating a low voltage, low current charger in terms of efficiency.
Also, it is the current that is split between the two cells in parallel. Some of these proprietary technologies are now pushing 6+ A, so even if you split it between the cells, it is charging smaller cells (2000-2500 mAh each) at relatively high current/voltage levels with a lot of loss. It will thus deteriorate both the cells equally fast, much worse than a single large cell, while charging quite inefficiently.
I wouldn't worry much about battery. I will prefer to go for large battery with fastest possible charging. Battery replacements are cheap. Mostly under 2K with labour charges at official service center for android devices. Not sure about iOS devices though.
Some people don't want to keep on creating e-waste because it is "easy" and "cheap", especially when it is easy to maintain a battery with a regular charger without much inconvenience if you are not lazy about it.
I don't know which oneplus charger you had but I've used everything from a Oneplus 3 charger to a Nord 2 charger and all of them have charged phones at normal speeds - not warp speed. Heck, I even charged an ipad (Air 2) using my Oneplus charger and it charged faster than the original Apple charger.
Does Oneplus follow standards, No. Do their chargers work with other devices, Yes. May not give you full speed but they do charge at normal speeds.
May be your experience is different but my experience with the 7T was exactly the same as in this thread. The third-party chargers were rubbish with the 7T when I didn't have the Warp charger.
Introducing our new OnePlus Community experience, with a completely revamped structure, built from the ground-up.
forums.oneplus.com
Similarly, my experience has been the same when using the Warp charger on other devices. Like below it is mentioned that it doesn't work with the Switch and in the same way it worked extremely crappily with all my other devices.
This night seem like a stupid question but can you charge other devices with it or will it fry it?
forum.xda-developers.com
People may hate it but Samsung, Apple, Google following the USB-PD specifications and not going overboard with marketing numbers is actually a good thing. USB PD devices also have a better fallback option with normal chargers than proprietary ones.