There was a time in the early years of wireless charging when Qi* was only a 5W standard, officially referred to as the baseline power profile (BPP). All phone chargers could charge all Qi 5W capable phones without issue. But the user experience at 5W was generally poor. So, it became important to move the power capability up to what is known as the extended power profile (EPP), which permits up to 15W of charging power. But what does 15W even mean?
There is only one thing that a phone manufacturer can do to maximize the wireless charging rate of a Qi compatible phone. They need to minimize the heating of both the charging coil inside the phone and the electronics associated with those coils.