Hermes Boots

Hermes Boots are an accessory that can be found in Underground and Underground Jungle Chests. Hermes Boots do not modify the players horizontal movement speed directly, instead they will give the wearer a speed boost. The boost is activated by running in the desired direction for 2-4 seconds (the time required to activate the "speed boost" is affected by movement speed modifying items). Once activated the wearer will run at a preset maximum speed (unaffected by movement speed modifying items) of 1.4*base horizontal movement speed. The effect will last as long as the wearer continues running in that direction without interruption. (Interruptions are colliding with an object or sustaining knockback).

Jumping while running will not cancel the speed boost, even if the Cloud in a Bottle, Shiny Red Balloon, and/or Rocket Boots are used. Falling onto a lower level of blocks will also not interrupt the speed boost. However, building up towards the speed boost only takes place when on the ground. Jumping or falling downwards will suspend, but not restart, the build up.

If used along with a Cobalt Shield, you will not lose the speed boost after getting hit by an enemy.

There are a number of ways to activate Hermes Boots boost even faster:
 * Boost speed can be reached quickly with the use of the Grappling Hook or the Ivy Whip.  This is done by grappling the ground before you (around 5-15 blocks) and jumping just as you are getting pulled forward. Also If you use Blizzard in a Bottle and jump nearly horizontal, at the end of the jump you will be going full speed without having to run for 2-4 seconds.
 * Doors can be used to instantly activate Hermes Boots. Grapple the middle of a door and then open it, continue running in the direction that you opened the door and you are now running at full speed!
 * Using the knockback from getting hit by an enemy is an impractical, but effective way of reaching top speed.

Trivia

 * This item has similar functionality to the Pegasus boots from The Legend of Zelda games.
 * Hermes Boots are a reference to "Hermes", the Greek messenger of the Gods, known for his winged sandals and incredible speed.