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?
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.
If you have lings with no speed and are facing speedlings, then make sure to get in a group and not a line. This will diminish the value of the speed boost because it will limit the effectiveness of the speedlings to get a surround on individual lings.
In general, the reason the speed boost wins is because your slow lings are traveling in a line, and are susceptible to being surrounded and owned.
To put it this way, the winner of 1 speed ling versus 1 normal ling will be determined by who strikes first (assuming no armor or weapons bonus). The winner of 6 speed lings versus 6 normal lings will be determined by how many micro battles have better odds for the speed lings. For example, if it can be 4 speed lings versus 1 ling, and then 2 versus 1, while the other 4 slow lings are still trying to get in combat, the odds are very high that the speedlings will win.