Well right now even though he did not use the power that he got from his bloodline because of the super physique you know that he got some Power boost and all this time he was using that. Well as you know that he just only had 240 stat on his agility, so he needed to use that to match the speed of those grasshoppers.
Also, when Allen uses the ghost step technique it increases his movement speed. It doubled his movement speed and because of this, he became much faster compared to those grasshoppers. But the main problem was that those grasshoppers could really jump high and he was unable to do that.
The moment those grasshoppers realize that they are in danger they will immediately jump in the sky or any other direction. Because of this as you can see it became very hard for him to kill those Monsters.
