Very slow charging after lightning port replacement

As @hootonberg said, using an aftermarket or non-genuine charging port is likely the cause for this. Did you make sure to clean the inside of the previous charging port properly with some fine-tip tweezers? I’ve found too often a “loose” charger port as you’ve described (slight pulls or angle changes will prevent charging) is usually the result of dust buildup in the port.
Even when people have told me they’ve cleaned it out I still manage to retrieve copious amounts of dirt and lint from deep within the port and after maybe 2-3 minutes of proper digging, the original port works just dandy! Worth a shot to clean the old port but if it is absolutely clean and empty (looks same as the new one side by side) and still not working, it likely was damaged and you should try to find a higher quality replacement charging port. It will be able to charge at the same rate the device originally did.