I don't know if you know how to edit the AI but nids need a look at.
Specifically their spore mine use and use of tyranoform. They use mines non-stop but never detonate them. They just send them back to their base and eat up their pop cap.
Tyranoform is used nonstop once they get the ability. While it makes the maps look cool it also can bring some serious lag and is more of an annoyance to never get a shot off from being knocked down.