Minecraft wiki entry for Mob_grinder, associated text: The downside to this trap is the slow rates of kills, since each mob needs about 10 seconds of suffocation before it dies. This problem can be fixed by having a row of pistons push together at once, suffocating multiple mobs in every smash. Another downside is the inability to handle spiders. You must find another way to sort out spiders, such as a cactus choke.