What defines the speed at which a motor is retracting and how can it be changed?
You instruct the printer to move from a certain X-Y position instructed by the previous move, to X=50 and Y=50. While moving at a feedrate of 100 mm/min, it will also retract 10 mm of filament (if the previous extruder distance was 0) during that move. If the movement distance is large, the retraction is slow. If you started from X,Y = 49.99,49.99 it would be very fast.

If you want a fast retraction, first move to a position, and than retract fast, so in separate commands. Do note that we usually do it the other way around: first retract fast and then move, this way there is less oozing of the nozzle.

To sum up, in your G-code command, the speed of retraction depends on the path of travel (the length and speed defined by the feed rate F). If it is fast retraction you are after, you should split the command into two separate commands.