Well, if you want action, if you really want action, you must play with an "x8" zombie spawn mod. Or made the changes to the xml yourself.
That's non stop zombie action. You can't raid a POI without an endless chain of zombies coming to greet you.
I removed after a little while because I don't want action!! (And because I thought that was crashing my game, but turned out that was a completely unrelated issue).
That was back into A19.
By the way, once you clear a place of zombies, let say, around my base, someone knows how long it will take until they respawn?