I still don't get why minecraft isn't properly threaded + mod threading (*cough* tickthreading *cough* Nallar). Servers would run like crazy ultra fast machines. If you can find 1.4.7or 1.5.2 MC + mods, you can see how server does not give a fuck if you spawn 3500 animals, or have 100k tile entities and server load under 25%... just because. Java is not that bad, Mojang coder team is bad.
So if you think minecraft can't run fast:
exhibit a: 4000 doors on the floor
exhibit b: 1000 pigs
exhibit c: 1000 pigs + 1000 cows
Not enough? 1000pigs + 1000 cows + 1500 ocelots
100 000 tile entities a problem? (tile entity: http://minecraft.gamepedia.com/Block_entity)
That is very good :D. But when you have over 100 mods it becomes a problem...