I just had a similar problem instancing nodes with static bodies. Not sure if it applies to your case, but in mine, changing the position of the node before adding it as a child solved the problem.
var child = Child.instance()
child.set_position(Vector2(x, y))
parent.add_child(child)
I guess Godot recalculates collisions on each child added and, by default, they would stack on 0, 0
, causing the lag.