At the end of each turn, Speed Boost raises the Speed stat of the Pokémon with this Ability by one stage.
Speed Boost will not activate on the turn the Pokémon with the Ability is switched in (even if it gains the Ability after switching in). It will activate if the Pokémon is switched in due to a previous Pokémon fainting.
Starting from Generation V, Speed Boost will not activate at the end of the turn if the user failed to escape from a wild encounter during that turn.
Prior to Super Mystery Dungeon, Pokémon with Speed Boost receive a boost to Movement Speed every 250 turns, lasting for the remainder of the floor.
In Super Mystery Dungeon, Speed Boost increases the Travel Speed of a Pokémon with this Ability by one after one turn of being on the floor, and again every 20 turns afterwards. The boost wears off after five turns.