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.
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.