[quote=@Zarkun]
<Snipped quote by Liotrent>

Yeah, Mules are beasts. But Zerg has been in the lead for a long time, since late HotS.
[/quote]

Yeah, that's mostly due to their great harass units, the zergling is one, early banes, maybe the odd muta harass. Zerg also expand like crazy while they do it, that's probably why Terran and Protoss have such a hard time keeping up.