[X Style (Pure) Q&A, Help & Troubleshooting] Tinkerer_:
There have been references published claiming that charging faster (higher current) shortens overall Li-Ion battery life.

Mechanism may be related to heat.

One thing the Qualcomm Quick Charge 2.0 (used in the Snapdragon 808) does is increase voltage at the charger from standard USB 5V, to 9V and 12V, for higher charge rates (power) at still-moderate current (to keep heat dissipation down).

I measured 1.1 to 2.3 amps at ~9V with QC 2.0 charger on the MXPE, with the higher current measurements at lower State of Charge (SoC). Have not seen 12V yet, but I only tried it down to 45% SoC (2.3 amps at 9V), I imagine it bumps up to 12V when the battery is discharged further, nearer to complete discharge.

This charger is rated for

5V, 4A

9V, 2.22A

12V, 2.5A

20V, 1.0A

So the max power fed to the battery would be 28W (12V*2.5A).

(This is the Power Partners PEAW30-12-USB, supposedly a 30W charger. So much for integrity in advertising.)

So the current is kept to a manageable level to control heat dissipation (therefore max temperatures), from the charger to (somewhere in the phone). But I believe that at the battery itself, more rapid charging (higher power) would still require higher current, because voltages have to be limited in the battery itself, so one would think heat dissipation (> max temperatures) would still be a problem in the battery itself. Does that shorten battery life?

The answer is probably: Who cares. Because: Li-Ion batteries have a 2-3 year life in any case, regardless of their service life or even if they are not used at all. They age and exhibit substantial capacity decline over time. Discharge/charge cycles hasten the capacity decline, but the battery is only good for 2-3 years, give or take, no matter what. And since aftermarket replacement batteries are inferior, unsafe, and stale, there is no reason to try to hang on to your phone for more than 2-3 years in any case. (Especially since the "non-user-replaceable" batteries can be a pain in the a** to R&R. The Moto X Pure 2015 battery is one of those. Some phones actually incur permanent damage to seals if the battery is removed/replaced - the Kyocera Hydro Wave is this way.)

You say "but you could replace the battery with an OEM battery". There are two types of OEM Li-Ion phone batteries on the market that an individual consumer can buy retail, when their phone is 2 years old or more: Used stale batteries (look up "reverse logistics"), and "new" (i.e. not put into service yet) stale batteries. Good luck finding a fresh, new OEM Li-Ion battery for your 2 year old or older phone (out of production for at least a year).

Been down this road before. Wasted lots of time and money replacing phone batteries after 2-3 years. From now on I'm going to stop coddling phone batteries, stop replacing them after 2-3 years, and just figure on a new phone every 2-3 years. It's the only way to get a fresh, new Li-Ion phone battery. (And get the phone right when it is released, like the MXPE this month. That way you are more sure the battery is fresh.)

I think everything in the wireless phone paradigm is increasingly heading that way anyway. Everything, and I mean everything, pushes the market to a 2 year product life cycle. Batteries last 2 years. Increasingly, batteries are not made to be replaceable. Carriers are changing networks so fast you need a new phone every 2 for that alone. New OS/SW overloads hardware older than 2 years. Displays may fade over a couple years. USB connectors wear out. Just relax and go with it. Marvel at the Qualcomm Quick Charge 2.0 (I am). You'll be happier with a new phone every 2 years. :)

Sorry for the long rant. :)