I acknowledge that performance is pretty bad, at times hellacious, but also I've not personally seen it tied to zombie count. Sure, yes, horde nights and such FPS dips - to be expected - but I've also seen a 100-count frame build in my queue go ting-ting-ting-pause-ting-tingting-pause as something else - with no zombies around - chokes the system every x-amount of seconds or so. And solar panels and pressure plates are HELLISH on performance.
That said, I installed a mod to add wandering hordes of 32 zombies and see NO performance drop from them (and it's a thing of beauty, btw to get stuck in a building with one of these hordes pouring inside: gadamn!)
My point being, I don't think zombie volume is as much as a hit as several other things that cause much greater lag.
Appreciate everything you are working on, and *looks at game counter* I now think 7dtd is my most played game in my game list