Disable "Charging slowly" warning
You will find a good explanation of charging slowly in answers to this question Android phone &quot;charging slowly&quot;: How to make it faster

  ...charge rate is fast enough for my purposes 



It is not what you feel, it depends on the power as you can see from Andrew's answer there. Depending on the power the toast is displayed


Try methods suggested there to improve charging power. If that doesn't help, there is nothing you can do to prevent the notification from showing up


Edit: What finally did work for OP was a thicker Charging cable as was one of the recommendations on the linked answer