Spawning massive amounts of zombies will increase the server's Ping and lower it's TPS (Ticks per Second) Rate. This can also happen in Single Player / LAN Mode.
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migrated
migrated
This might be a duplicate of MC-17630, if it is causing server lag (Even in SSP). If you're having FPS troubles, dunno then. :/
kumasasa
Duplicate of MC-17630
The issue does not occur on 1.6.1 i have tested this with the help of 2000 zombies (<3 MCEdit) and the issue has not shown up. i loaded the same map in 1.6.2 my normal 330 fps was 40 fps