This move effect may be in need of research. Reason:How the Ability functions in link battles between Japanese DP and other Generation IV games, effect in Pokémon Battle Revolution You can discuss this on the talk page.
Slow Start causes the Attack stat and Speed stat to be halved during the first five turns in battle. This counter is reset if the Pokémon is switched out.
In the Japanese version of Pokémon Diamond and Pearl, removing Slow Start does not undo the Attack and Speed drop; it must still wait five turns. This was changed in localized versions as well as in Pokémon Platinum.
In Pokémon Rumble Blast, Regigigas has the Special Trait-equivalent of Slow Start: Slow-Starting. Instead of going into full strength after an amount of time, it has to defeat 10 Pokémon to go to full strength.