Stefany,
If your iPhone 11 Pro, charges at an acceptable rate to 80% then slows down until it reaches 100% then that is normal behavior. It is also part of the battery optimization. Go to Settings-->>Battery-->>Battery Health and then check to see whether Optimal Battery Charging is turned to ON. You can see the at the bottom it explains what Optimal Battery Charging means. Hope this helps. You can turn the Optimal Battery Charging to the OFF position if you want to charge the battery all the way up to 100% without slowing down after 80%.
Axel F.