So I've had my Samsung Galaxy S8 for a few months now and aside from the extremely delicate glass, easy scratching and finger print scanner glitches the most annoying issue of all must be the slow charge. I keep my things in good condition, if I let my phone get wet, I dry it the first chance I get (which is usually not long after), I've never tried jamming anything into the ports or had the phone in salt water and yet my phone takes nearly 4 HOURS to charge from 35%. I reported this and a Samsung rep basically told me try using a different cable and if that doesn't work to get it fixed. I swear if I have to end up paying more on top of the high cost I paid I'll be getting rid of this phone as soon as possible.
Your phones not actually fast charging thats your main problem and if it's a technical issue they should cover the fix, are you using the charging equipment that came with the device? I can't attach a picture through comments but where it says "Charger Connected" it should read "Fast Charger Connected" you would also see that in the notification pull down it should read "Fast charging" where yours will probably say "charging". Most of the time this isn't due to the cable but due to the AC Adapter piece, the one out of the box is a fast charging one, if you can get another one of those to test then it might be worth checking to see if it's just that piece.
Edit: Also, just found this in the setting but go to Device Maintenance - Battery - Drop down for advanced settings - and check to make sure fast charging is actually on
Ya my thought is either the cord may be frayed somewhere causing alot of resistance between the outlet and your phone.... Or perhaps you were unlucky enough to get a faulty usb-C port. Have you tried checking the integrity of all your ports? You can literally do it inside the samsung members app on the front page.
I have and it doesn't seem like a good test, it instructs you to test the USB port, I plug it in (still slow charging) and the app just says "good". I wouldn't be surprised if the cord is frayed, Samsung and apple alike have poorly made chargers in terms of durability but I treat my stuff well and haven't snagged the cord so that'd be unusually quick wear.