Note : Please Use Original 15w Samsung Charger Which Comes in Box with Galaxy M20 If you are using any other charger or cable its obviously gonna charge slowly
Solution: Unplug the Charger and turn on Fast Charging Cable mode in Settings>Device Maintenance>Battery>Advanced Settings >Fast Charging Cable. I think you got this mode turned off but if you already got this option turned on but still phone charges slowly then reboot the device and just don't use the device during charge because when you are using the device during excessive heat is going to be generated and fast charging already heats up the device due to which in order to prevent itself from melting the device would disable fast charging mode and use standard charging option Instead which would very slowly charge the Big Big 5000mah battery, So don't use device while you are charging it with fast charging
Edit : I forgot to mention that mobile phone batteries require moderate temperature to charge effectively so if the battery touches any of two extremes (Hot & Cold) the charging rate decreases and i have tested on my Galaxy M20 Battery temperature remains at 32 - 36°C during Charging but if i do other heavy stuff its temperature even reaches 40°C and beyond so if you are playing PUBG (Though Lagging) alongside charging your then you are just killing your device and your phone is not gonna charge even 1% in half an hour
