First thing to try for any phone charging slowly, especially if the phone says something about check the charger connection or anything within the ballpark of super slow charging is: get a NEW charging cord. USB cables go bad over time from repeated flexing, twisting, bending etc, and the wires inside them fray and break. Frayed wires can't carry as much amperage and broken wires carry no amperage.

FIrst thing to try would be a brand new USB cord. Most of the time slow charging is from the cable itself going bad. If a new cable and a samsung wall adapter (adapter doesn't have to be new, just has to work) doesn't fix the slow charging, then you've likely got a problem inside the phone.