It seems like Samsung's all-new Galaxy S8 and Galaxy S8 Plus flagships are not using their fast charging feature while you're using them, which is quite interesting. This information comes from Phandroid, and this 'problem' has been present in the company's previous flagship devices as well, the Galaxy S6, Galaxy S6 Edge, Galaxy S7 and the Galaxy S7 Edge have all performed the same in this regard, read on.
The Galaxy S8 and Galaxy S8 Plus flagships are taking advantage of the company's bundled in Adaptive Fast Charger, and are charging at either 9V/1.6A or 5V/2A. Now, in plain numbers, the Galaxy S8's 3,000mAh battery can be charged from 0% to 100% in an hour and 37 minutes, but if you use the device while it's fast charging, it'll take it quite a bit more time to charge up, especially if we're talking about prolonged use here. The moment you turn the Galaxy S8's or Galaxy S8 Plus' display on, the phone will stop fast charging, and will continue using regular charging speeds, quite are quite slow, needless to say. Now, it's worth noting that the Galaxy S8 will say that it is fast charging on the display while you're using, while it's not, the moment you start using the device you're losing its fast charging capabilities, which is why that notification should be one once you turn on the display, or at least some sort of a warning should be present.
As already mentioned, it takes the device 1 hours and 37 minutes to reach a full charge using Adaptive Fast Charging, well, if you're using the device while it's charging (all from start to finish), it will take you 2 hours and 51 minutes to get it to fully charge, which means that it'll take the phone almost twice as long to reach a full charge. Now, phones that are using Qualcomm's Quick Charge 2.0 or 3.0 solution, do not have such issues, times might differ a bit (a couple of minutes), but you'll get approximately identical charge times. According to the source, it took the ZTE Blade V8 Pro 1 hour and 32 minutes in order to charge up its 3,140mAh battery from 0% to 100%, and we're talking about Qualcomm's Quick Charge 2.0 fast charging solution here. Same thing applies to OnePlus' Dash Charge which the OnePlus 3 and OnePlus 3T are using, Dash Charge will kick in no matter if you're using the phone or not.