Have you been able to test other charging cables and other power sources to see if the charging rate improves? It is recommended that you use a charger that is plugged into a wall socket instead of using a computer (and USB cable) to charge your device.
Another thing that you can try is to reboot your phone in safe mode to see if the charging works better there. Safe mode is a troubleshooting mode where all apps downloaded from Play Store is temporarily deactivated and in that way, you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.