I play PZ on MP and when there is a group of zombies, the FPS drops drastically. In the center of the LV with a large horde, FPS dropped to zero.

It was most visible when there were dozens of zombies lying on the empty road. Reaching the stack, the FPS value dropped sharply.

The other player also on MAC is experiencing the same problem.
PC players see that there is less FPS, some lag but the drop is not drastic.