Minecraft 1.4.2 Lag Issue

After downloading minecraft 1.4.2 i have been having big issues with lag in farm area of my world only. on 1.3 my game worked fine and had a consistent 30-40 fps in all parts of my world no matter what i was doing in it. But now my general fps are down to 20-30 and when ever i go into the crop farm area my fps drop to 18-20 and whenever i break a block it drops instantly down to 10 fps or lower then goes right back up to 20. Can someone please help me its making it difficult to play the game.