Karkat is fairly fast. He needs to be, because fighting people based on pure strength is not the kind of tactic that works for trolls his size. And when he truly needs to, he's capable of working silently. The night and the droning of the dokibeasts only help him in this.

So when a five-foot-tall blur slams into Sherlock, knocking him to the ground, there is absolutely no warning beforehand. The attack itself is barely warning for Karkat yanking Sherlock's arms behind his back, tying the rope around them quickly.