Apple 15w wireless charging speed is not fast, his fast charge power is 18 watts, not 15 watts 18~25 or 22 watts, belongs to the pd fast charge protocol power, and 15 watts and 5 watts should be no difference, Apple then it has a charge protection chip, for some high power it will reduce the frequency to 5 volt a can not be accepted, Therefore, the charging speed may be the same as 5 watts.
The Apple xr takes two hours to fully charge with a 15w wireless fast charge
About 1 hour and 20 minutes full