The speed of cards determines how quickly their turn comes every round and strongly affects the hit/miss ratio.
Speed is not a fixed attribute. It often changes when a monster levels up and it is affected by the Swiftness (+1) and Slow (-1) abilities, and also by some summoners.
Also, in the Reverse Speed ruleset, everything is reversed in terms of speed. The slowest monsters become the fastest each round (and harder to hit), and the fastest monsters have their turn at the end of every round, instead of the beginning, as they normally would.
