The creature’s speed becomes 2 if it was higher than that. This speed applies to all of the creature’s movement modes (walking, flying, swimming, and so on), but it does not apply to forced movement against it, teleportation, or any other movement that doesn’t use the creature’s speed. If a creature is subjected to this condition while it is moving using any of its speeds, it must stop if it has already moved at least 2 squares.
The creature cannot benefit from bonuses to speed, although it can use powers and take actions, such as the run action, that allow it to move farther than its speed.
A variety of different effects can cause a creature to become slowed. A creature subject to this condition might have taken an injury to the legs (or other limbs used for locomotion), or it might be entangled in something. The creature could also be poisoned, beset by wracking pain that hampers its movement, encased in frost, or slowly turning to stone.
Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 223, Rules Compendium, page(s) 233, Monster Vault: Threats to the Nentir Vale, page(s) 125.