Why do zerglings with speed beat normal zerglings?
With Metabolic boost "speedlings" become a lot better then the un-upgraded zergling. This is because they are faster. But when matched up with even number of un-upgraded zerglings they always win. If the metabolic boost upgrade only increases speed, why does it seem to increase rate of attack and DPS?
Solution 1:
The speed upgrade allows the zerglings to do what they do best - surround - faster.
The more zerglings hitting your zerglings, the quicker they die, and the faster zerglings can surround other zerglings, the more zerglings are hitting your zerglings.