like STR and CON we can infer it doesn't mean any one thing but is some aggregate representation of peak movement speed, reaction speed, dexterity, etc.
And a concrete example would be CM3 talking about the differences between FSN Lancer and Rider's equal rank of A. Cu Chulainn as the fastest Heroic Spirit in FSN has the higher peak movement speed but Medusa can keep moving around quickly for a more sustained period of time.