Mud slides are a fast type of mudflow that are ussually up to 50 mp/h. Now mudflows are ussualy less depending on what caused it. Some are only a few meters a day at top speed depending on how much of a slope and how long the slope is. They also depend on haw thick the mud is.
If there was a recent fire it adds many triggers to mudflows. Firstly the ash will move into the ground and when it gets wet it gets extremely slippery. Next the lack of plant growth will be a lack of roots to hold the soil in place.