I am not a scientist to answer this question but here is what I have found through my research although I doubt one can give you the exact speed, as there are many factors. Further testing the speed of cloud I guess it would require something to test it, it is out of my small brain. But here is something that might be useful..
Clouds move with the environmental winds. High cirrus clouds move with the jet stream at speeds near 100 mph at times. Thunderstorms usually move at 30 to 40 mph but can move as fast as around 60mph.