Awesome way to lag the hell out of your server, create a zombie xp grinder with a zombie dungeon you find.
Updated my server to 1.6.1, in 1.5.2 there was no lag issues at all with the server, update to 1.6.1 and the server is going extremely slow.
Put a profiler on it, and it turns out the server is spending most of its time doing 'net.minecraft.src.EntityZombie.onUpdate'.
Did Mojang actually think of this issue before creating these social zombies? Maybe make it so that if the zombie was spawned by a spawner, it doesn't call out for friends to come and kill you.
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Profiling is available @ http://www.red-blocks.com:23000/stack
On further looking, it isn't the zombies calling for help, it is the new path finding capabilities where they can detect up to 80 blocks away causing the issue.
Screenshot of the profiling available here: https://docs.google.com/file/d/0BwBrPN59-fKNbGNNMG1UcEVYVjg/edit?usp=sharing