soil creep is the slowest form of mass movement. the expansion and contraction of the water within soil is the main cause of it. wetting causes the soil to expand and to move downslope under the influence of gravity.
Oh yes! They are definitely a type of mass movement - LOTS of mass moves in an avalanche. Having seen at least a few first hand (and helping dig by hand/shovel through a road blocked by one) I can confirm there is a LOT of mass moved.
Orbital speed is determined by the mass of the body and the distance from the sun. Given its extreme distance, Pluto has the slowest orbit. However, Pluto is no longer considered a planet, but instead a planetoid. Therefore Neptune now has the slowest orbit.