Tn most cases, a phone is charging slow because there’s something wrong with either the phone itself, the charger or the way you’re charging the phone. Let’s talk about each situation in which your phone takes too much to charge and see if there’s a fix for it.

1. Check the cable (self-explanatory - if it's damaged or broken you need to change it)
2. Check the adapter (always try to use the original one)
3. Check your power source
4. Check the USB port(yes, it gets dirty quickly)
5. Check the phone’s battery (if you can change it for just a few hours just to see if another's (Similar) phone lasts longer)
6. Check the background apps

https://www.unlockunit.com/blog/my-phone-is-charging-slow-what-should-i-do/