Speed

Speed, abbreviated as SPD, is a type of internal variable that represents how many units Mario moves per frame. There are many speed variables, such as Horizontal Speed (HS), Vertical Speed (VS), X/Y/Z Speed, Sliding Speed, and X/Z Sliding Speed. These variables do not always represent how much Mario actually moves per frame; for example, when something is in his way, so he has horizontal speed but isn't going anywhere.

Gravity

Mario falling at Terminal Speed (M75) in PSS

When airborne, Mario's vertical speed decreases by 4 units every frame due to gravity, and is capped at -75 units per second.

Speed Conservation

The speed variables are normally updated every frame, but sometimes this is not the case. When outside of water and not sliding, Mario's sliding speed is constantly updated to whatever his horizontal speed is, and when in water, Mario's sliding speed isn't updated at all. Therefore, horizontal speed can be conserved by going into water and entering a sliding state on the same frame Mario exits (for example, by pressing Z to crouch) to activate his sliding speed.

Some other examples of speed conservation are Vertical Speed Conservation and Vertical and Horizontal Speed Conservation.