I love this mod. Been running this on my test server and here are my observations:
Initial server settings - Insane difficulty, walk-walk-sprint-sprint, 80 zombies plus vanilla walkersim settings
This was very very intense. After a few minutes there were zeds everywhere. We had to continuously run to avoid getting overwhelmed. It was difficult to do much non-killing activities due to the constant throngs of zeds.
Tried a different set-up.
Second server settings - Warrior difficulty, walk-walk-sprint-sprint, 64 zombies plus vanilla walkersim settings except that I lowered the Population density to 70 from 120.
Still very intense, but just slightly. A bit easier to kill the zeds so I might up the difficulty to Survivalist. Still have to move a lot to avoid getting overwhelmed. I did see sleepers inside buildings so they appear to be working fine on my server.
Performance has been silky smooth with a lot of zeds on screen and they appear to exhibit a nice range of behavior. Some wander aimlessly while others seem to head right to me once the "notice" my presence. As someone else mentioned above, is there a way to allow the player to temporarily "clear out" an area to be able to have a tiny break from the zeds.
Picture below is with a Population density of 70. There are 9 zeds in front of me and another 6 to 8 on my sides and behind me.