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 Pokemon is switched out.
If this Ability is replaced due to a move like Skill Swap or Worry Seed or suppressed by Gastro Acid, the Pokemon’s stats return to normal.
In the Japanese version of Pokemon 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 Platinum.
Outside of battle
Slow Start has no effect outside of battle.
Pokemon Mystery Dungeon: Explorers of Time and Explorers of Darkness
When the Pokemon enters a dungeon floor, its Movement Speed is reduced temporarily, but its Attack is not affected.
Pokemon with Slow Start