I currently have 3 worlds that I have been working on. World 2, my main world has recently been lagging to the point of freezing the program (I use the stock minecraft client) completely, either just after I log in, or about 5 seconds into gameplay.

It happens in both windowed and fullscreen, online and off, and in the browser. World size is 9.2mb, but I'm sure that doesn't have anything to do with it. I reinstalled java, and did the "delete the bin folder" trick... I can't believe it has anything to do with not enough available ram ( I have 6gig)... Does anyone have any clue what could be going on?
Before anyone accosts me for not searching, I have. Several times.