I would think running on concrete downhill in flats would be faster than downhill on mondo with spikes.
I once pushed a Plymouth Champ out a snowbank in the middle of the country soused like you could not believe, my best friends and I, 2nd and 12th in 800m at State Meet their senior seasons were out skiing on the back of the car. I don’t reccomend this by the way. Just an example of mind over matter, I weighed about 120lbs at the time and it was a plymouth Champ.
as for 12m/s, that is faster than the Human body can possibly withstand over any distance greater than a couple of strides. Downhill running requires significant skill development.