Minecraft wiki entry for The_Wild_Update, associated text: Added the Swift Sneak enchantment.
This enchantment can be found exclusively in chests in Ancient Cities.
Cannot be obtained by fishing, enchanting, trading, or other loot chests, similar to Soul Speed.
Has a total of three levels.
Available for leggings and boots, but does nothing if applied to boots.
Allows the player to walk faster while sneaking.
Has an increment of 15% per level, for 45%/60%/75% of your walking speed with level I/II/III, respectively.