Metabolic Boost enhances the speed of Zerglings, greatly enhancing an already fast unit's natural advantage. Zerglings with this upgrade are commonly referred to as Speedlings. It's a nearly essential upgrade in every match-up, as it gives your Zerglings a greater ability to surround your enemy's units and retreat when warranted.
Combined with Zerglings' natural high speed and the speed boost given to Zerg units on creep, Metabolic Boost can be useful in defending Zerg Drones from Hellions attempting to kill Drones or early Zealot pressure.
When Metabolic Boost finishes researching, wings appear on the backs of the Zerglings.