I guess only time will tell but battery tech has moved on
Tesla use a fast charge in their cars, they wouldnt use it if it would be at the detriment of the life cycle, time will tell
I'm not sure even Tesla would agree 100% with you, especially if charging in a hot climate (Over 85F). Cooling is crucial to battery longevity.
Heat dispersal is crucial to a fast charge in current lithium batteries (there are other tech that would make that less of an issue in the near future). Not sure how Samsung handled it here.
So, if you charge mostly in a controlled temp area and you're phone is not enclosed, or on a soft surface, charging fast may not be too much of an issue, up to a point. If you phone becomes very hot when charging, well it will be an issue...
Another issue is discharge depth (going to 0% often, that's why phones are shut down when approaching zero) and over 100% charging (this is bad and most phone batteries indicating 100% are in fact not really at 100% to prevent this). A fast charge from 10% will create much less damage to the battery, than a fast charge from zero.
Another thing that has impact on battery durability is discharge rate. In cars seemingly discharge rate and battery usage (how long, how you drive your EV car) has more impact on battery life than charging rate.
Playing fps shooter/racing video games on your phones in hot weather (especially while charging) will certainly make your battery last less than using it for web browsing, or taking calls in cool weather.
But, hey its only a phone, lets replace the battery for $30-40 bucks when its dead. That's what I do.
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PS: The Iphone 6 with the Ipad charger is not really a slow poke at about 2h (0-100%). Most people aren't at 0%, so you're looking at charging times in the 100-90 minute range if your phone is used heavily.