I have had this problem recently but I purchased a new cable which enabled fast charging but today it is back to slow charging, anyone got any advice?
I'm guessing you've definitely got fast charging enabled in Settings > Device Maintenance > Battery > 3 dots > Advanced settings > Fast cable charging? (Just checking to make sure.)
If so, any damage or obstruction (like dust or dirt for example) in the charging port? Or any firmware or apps updated recently just before this started?