I have an i7 3.5GHz processor
32GB RAM
NVIDIA GTX 1080
and I'm getting around 35-45fps while standing still in the middle of an open field. During horde nights, I change my settings to the lowest possible settings across the board, and I range between 3fps to 25 fps. If pathfinding is the reason why the framerate drops that drastically for an $800 video card, then there's wildly extreme problems with the way the pathfinding is written. It would have to be on the level of trying to run the entirety of Crysis with each zombie.