I often hear of "Speedlings" in Starcraft 2 but what do they mean exactly? And why are they so good?
How can I hinder the enemy from building extensions with Speedlings?
It's the name given to zerglings whose movement speed has been increased due to the Metabolic Boost upgrade.
There are a lot of abbreviations in Starcraft 2 like Speedling for Zerglings with speed upgrade found in this complete list of all abbreviations in Starcraft 2
Speedlings are one of the units which come from the original starcraft. As mentioned above, they are called like that after the metabolic boost upgrade, which significantly increases their speed. It is a must have upgrade in every match up, since zerglings without speed are pretty much useless when not outnumbering the opponent at least 3:1.
Another thing which should be noted is that an alternative to the name is a "Crackling" after the tier 3 attack speed upgrade has been researched.
Other than direct combat, seedlings are often used for counter attacks, scouting, controlling the Xel'naga watch towers.
In the late game, they aren't a preferred unit in the main composition due to their low sustainability, however they are often the first units which are built after the initial engagement since they are very mobile and they can deliver a fast hit on your opponent.