Noticing hat simply keeping the phone elevated a little (to provide a gap for airflow across the back of the device, improves the charging performance, and battery temperature.
It would be interesting if the smartphone came with a specially designed rack to provide that air gap, and keep the temperatures lower.
This is safer than placing the device face down on a surface, which will risk scratching the screen.
Currently when charging while resting on a flat surface in a room with am ambient temperature of 83f (28C), the battery can reach temperatures of 46C while charging, it doesn't seem to get hotter than that, thus I assume the charge rate is slowed to let it cool off.
With an air gap, and a slight amount of airflow (fan in the room), the temperature can generally remain under 110f (43C) throughout the charge process.
I use some cardboard with the corners raised in order to provide an airflow channel under the device, in addition to some passive cooling from the fan in the room.
I've tested this and there seems to be some improvement. Of course this is with a phone that has the miniUSB.
I have tested on USB C as well and the result is minimal but when the phone is completely drained there is a difference as fast charging seems to elevate the temperature.