The problem isn't density, it's respawn. It's also server dependent (Low/Med pop servers are the hardest hit) Zombies are currently spawning at an incredible level. My Clan recently killed 100 zombies, and had 2 vehicles go down in what turned out to be a fruitless endeavor to simply clear the area around our base. 100 zombies killed.... and we were still surrounded. This was after I personally spent 30 minutes with 100 rounds of shotgun killing so, so many zombies.... I eventually died when they attacked me through the foundation. Just be aware that just because your server isn't affected, doesn't mean others aren't having their own issues. It's real.