I'm experiencing an important lag when there's a lot of mobs around us. Actually, I started a private server (on local network, on a regular gaming pc) in 1.5.2 without any problems. We set a little base around the spawn chunk with villagers.

The problems came sinds we upgraded to 1.6 : when the zombies came around the NPC's, the lag became important. Note that it isn't a fps drop, but a delay between the game and the server, as if it was overloaded. In fact, a simple block takes around 10sec. to desapear when broken and the night and day cycle is way longer than usual.

We tried to fix that by playing solo, increasing the ram amount allocated to Minecraft. Nothings changed.

The only solutions are to downgrade to 1.5.2 wich is working fine or to play in peacefull.

Regards, Boris