It's common knowledge that smartphones charge faster if they're left sitting idle. Using a phone while it's charging won't get you the fastest charging times possible, and with Samsung devices, this holds even more weight. As Phandroid points out, the Galaxy S8 and Galaxy S8+ will not charge fast when the phone is actively used while charging, or even if the screen is on. The fast charging notification will continue to be shown in the status bar, but the devices will charge at regular speeds.
This has been the case since the Galaxy S6, and since all Samsung devices continue to use the same fast charging technology (Qualcomm's Quick Charge 2.0, or Adaptive Fast Charging as Samsung calls it) even today, the Galaxy S8 and S8+ suffer from the same limitation. Phandroid tested things out on the Galaxy S8 – the phone took only 1 hours and 37 minutes to go from 0 to 100 when the screen was off, while charging time went up to nearly 3 hours (2 hours 51 minutes, to be precise) when the screen was kept on at all times during charging. That's only for the S8 with its 3,000 mAh battery, and the S8+ will no doubt be even slower.
It's rare for someone to use their phone for the entirety of the charging period, especially when they are topping it up from 0 to 100. But it's still good to keep in mind that you will see your phone charging faster if you just leave it be and refrain from turning the screen on too much. Why Samsung has the limitation in place is unclear. It could be a way of ensuring the device doesn't overheat (fast charging tends to heat up a phone more than regular charging), but whatever the reason might be, it's clear that fast charging works best on the company's flagships when they are left sitting idle.
