I recently upgraded to a 23 Ultra from my S10+ however I've noticed the same problem with both phones. If I select fast charging in settings, the phone charges fine however if I deselect fast charging the starts out with a reasonable charge time of like 1.5 hours from 15 percent, and then about 2 minutes later jumped to about 3.5 hours. I plugged it in before I went to bed and woke up 7 hours later to find the phone at 88% saying it still had over an hour left to charge. I had the same problem with my S10+ where it would start out fine and then seemingly get slower over night to the point that I would never have a full charge even after 8 hours. I have an original Samsung brick that came with my S10+ that I am using.
S10+ Has a less capable brick. Please purchase a variable power usb c brick, preferably from the Samsung site, of 15w or higher. And leave fast charging on, it doesn't harm your phone in anyway.
If you are still experiencing the slow charging, we recommend having the phone evaluated by a certified technician. You can use the following link to find a list of service centers in the U.S. that can do the repair: http://www.samsung.com/us/support/service/location or by contacting customer support at 1-800-726-7864.
