I'm just guessing here, but when you select slow or fast mode on Microchip SAM MCUs there is an observable change in the pin series resistance. My best guess is that the idea here is to have an RC filter formed by that variable resistor and the line capacitance.
Again, just guessing and would be interested to know the details myself.
My observation method may not be perfect either. All I did is set the pin as an output driven to 0 and measure the resistance from the pin to ground using a multimeter. Since there is only two settings in SAM MCUs, the results are hard to interpret. May be you can do the same measurement on the STM32?