The first unique characteristic separating Apple iPhones from other smartphones is that Apple is using a custom CPU architecture that differs a lot from those of other vendors. It’s plausible that the architecture is able to power down and power up in a much more aggressive fashion compared to other designs and as such has stricter power regulation demands. If this is the case then another question rises is if this is indeed just a transient load issue why the power delivery system was not designed sufficiently robust enough to cope with such loads at more advanced levels of battery wear? While cold temperature and advanced battery wear are understandable conditions under which a device might not be able to sustain its normal operating conditions, the state of charge of a battery under otherwise normal conditions should be taken into account during the design of a device (Battery, SoC, PMIC, decoupling capacitors) and its operating tolerances.
If the assumptions above hold true then logically the issue would also be more prevalent in the smaller iPhone as opposed to the iPhone Plus models as the latter’s larger battery capacity would allow for greater discharge rates at a given stable voltage. This explanation might also be one of many factors as to why flagship Android and other devices don’t seem to exhibit this issue, as they come with much larger battery cells.
And a fair point from Tsai himself:
Lastly, how long should we expect a phone to last? Especially one like the iPhone X? With higher prices, the move away from carrier contracts, and diminishing returns for the camera and other new features, it seems natural that people will want to keep their phones longer. But that seems totally at odds with the design and battery choices Apple is making.
On that note, there seems to be some confusion about whether fast charging impacts long-term battery life and degradation. Rene Ritchie says that it does; John Gruber asked Apple and they said it doesn’t.
In hindsight, I think I was too nice in my first piece on this. What I wrote yesterday was that “I don’t think they communicated this very well”. What I should have written was that they didn’t communicate this at all on the record, and that’s not acceptable. I still think that reducing CPU performance is a reasonable choice to make, but perhaps it’s a choice they had to make because of other decisions, like the balance of battery capacity to maximum CPU power draw.