Yeah even though thats the point, it isn't very efficient if it crashes a device.
If it spawns a whole bunch of zombies at once it is bound to make the device lag or crash. Instead how about making it spawn 4-5 zombies every certain amount of ticks?
Would be less laggy and efficient. << Those are my ideas for improvement.
Also can you use a different skin for zombies? I hate the green looking ones. >.>