The Maths

The time it takes to charge a phone can vary depending on several factors, including the phone’s battery capacity and the phone’s current battery level.
For optimal wireless charging performance, it’s recommended to use a relatively thin and non-metallic phone case. Thin cases allow for better transmission of power between the charger and the phone, leading to faster charging times. Additionally, removing the case entirely can sometimes improve charging efficiency, especially with thicker cases.

Base on my maths with a 30min timer

Galaxy S20 OFF

50% => 64% and 28.00% Increase

No case 

Galaxy S20 ON

48% => 61% and 27.08% Increase

No case 

Galaxy S20 ON

38% => 48%  and 26.31% Increase

With a case on

Note: 
In general, using the original charger that came with the phone is usually the fastest way to charge it. However, the charging time can range from about an hour to several hours, depending on the factors mentioned above.