Arcanum of Rapidity

The Arcanum of Rapidity is a enchanted gem that will add a haste effect to either a head or leg item. This effect will not stack with other enchantments on the item.

Source
The Arcanum of Rapidity is a quest reward from the quest Libram of Rapidity which starts with Lorekeeper Lydros in Dire Maul. To complete the quest, you must turn the following items:
 * 1x Libram of Rapidity
 * 1x Pristine Black Diamond
 * 2x Large Brilliant Shard
 * 2x Blood of Heroes

As a quest objective

 * The Savage Guard - Arcanum of Rapidity