I feel like people aren’t really thinking this through. Fast charging matters more when your phone is tethered to the wall. If you are using an external battery like this to supplement your phone’s battery, it doesn’t make a lot of sense to charge as fast as possible; it’s just creating more heat and shortening the useful life of the phone’s battery more quickly, and the phone isn’t tethered to anything. This was probably exactly Apple’s logic, since it charges the phone at 15W when plugged in.
I went back and read reviews of mophie, anker, and a couple other magnetic chargers, and all of them ding those chargers for not being "true" MagSafe because they only charge at 5 or 7.5W.
Interesting watching folks now trying to justify that behavior when the charger comes from Apple.