I won't disagree that performance can increase with better coding but it's what we have. Remember this started out as a small studio.Performance is only an issue if your game is badly designed. Some game have thousands or hundreds of thousands of individual units fighting each other at the same time because they know how to get around the usual problems, just like game with SpeedTree can have tons of trees and stuff. And why do you need a single player to tackle a horde exactly? Nobody ever does that (except for this game of course). If the zombies acted as they should, which means not destroying your base, it would be very "the walking dead-ish" and very cool with the horde being super annoying, but easy to run away from or get through in disguise. The way zombies work in the game right now is an absolute no from me in terms of more zombies. If we want more zombies (which I want), we need to make them more stupid and slow as well.
I think it's better to accommodate what is true rather than what we want to be true for this as refining the code from many alphas would likely take a very long time and it's not garunteed to offer a ton of performance benefit.
I imagine a lot of pushback for making the zombies generally weaker while tanking performance.
What we have could use some tweaks as I mentioned with certain POIs having more zombies or a zombie slider in the options but I don't think it's worth much more than that.