When i enter in some very specific areas of some of my worlds (I'm afraid most of my worlds if not all have this problem), in both solo and multiplayer servers, I get huge FPS drops for a very long time (most times the only solution is to close minecraft). Drops do not continue when i leave these specific area. There is absolutely no buildings, no redstone or crazy number of entities or anything like that in these areas. I deleted the lagging chunks with mcedit, filled it with air, blocks etc. and this had no effect on the issue. These areas seem to be placed near the spawn chunks. I use to build some redstone and farm mecanisms on the spawn chunks but with very little redstone and entities (about 80 cows, no redstone clock, only one bud switch 14 block long piston sugar cane farm). I don't know if those buildings have something to do with the problem but i have absolutely no issue within the spawn chunks and near the farm buildings. Also lags seemed to happen before i built those things.