Charging a Lithium battery at more than its capacity (IOW, for the S8+, more than 3.5 Amps, will destroy it very quickly - and Samsung would have to build a few new "battery replacement centers", so they're most likely keeping it at 3.4 Amps or less. And constantly charging it at 3.4 Amps (which fast charging doesn't do - it tapers off starting at about 50%) won't harm it at all. A 3,500mAh battery is designed to accept a 3.5 Amp charge. (It's called a 1C charge - 1 times the capacity of the battery, and that's been the maximum safe charge for lithium batteries since they were invented. They're working on faster charging lithiums - imagine a car running on lithium batteries and you drive cross-country. You'd be watching hawks make circles in the sky for a couple of hours while your car recharges.
But anything under 1C these days is safe. And a full charge at 1C, from 0% (which you should never do - it kills the battery quickly) to 100% should take about 90 minutes. So if you're putting "half a day's charge" into a battery in 30 minutes, you're far from exceeding 1C.)