Thanks! In hindsight I think trying to spawn individual zombies may have been a bad idea. Having a bunch of them trying to find their way across the map is pretty resource-intensive, whereas I could probably have got away with spawning a single "horde" object and then replacing that with a bunch of zombies only when it got sufficiently close. I couldn't guarantee a quick player wouldn't just scoot away from those, but it would mean far fewer enemies plotting a route through the entire maze of buildings.