With Metabolic boost "speedlings" become alot 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?
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.