Drizzlewood mine defusing event does not scale

As the title says - the Drizzlewood Coast meta mine defusing event does not scale (or at least does not scale well). Unclear to me if this affects both metas, since south has much smaller groups at the objectives. But the north one can't stand up to the zerg - it is completed in literally < 30 seconds. If you have enough time to defuse 1 mine you're lucky. This makes the associated achievement extremely tedious, especially since you probably won't unlock it until you have already done the map many times.