I run a vanilla server with snapshot 14w18b, and about a week ago we started experiencing severe framerate drops and high CPU load in certain chunks.

What happens?
When entering the affected chunks, after a few seconds the CPU load rises significantly, and the framerate drops to 1 frame every 3 seconds. Once the player leaves the affected chunk, the framerate recovers to a normal value, and the CPU load drops, too.

The affected chunks are not special in some way: They are not full of hoppers, item frames, or crazy redstone contraptions. In fact, I can't recall that anything was built in these chunks during the last few months, and certainly not right before the issue appeared for the first time.

The issue affects the CPU load of the server, too. But the server does not provide any error messages besides "Warning! Can't keep up [...]".

Affected versions
We've been running snapshot 14w18b since it was released, and the issue did not appear right away when we switched to this snapshot. (We (used to) walk through these chunks quite frequently, so I'm pretty sure about this point.)

I started trying other snapshots and stable releases to see which versions are affected:

• 1.7.4 is not affected
• 14w18b is affected
• 14w26b is still affected
• 14w27b is still affected
• 14w28b is still affected

SSP/SMP
I downloaded the world and ran it as a singleplayer world (again with 14w18b) and the issue did not appear. Even opening it as a LAN world did not help to reproduce the issue. This made me think that it could be related to my server in particular, so I uploaded my world to a fresh, clean, vanilla environment, but the lag was still there.

Mobs, entities
I set the difficulty to peaceful to see if mobs or their pathfinding could be the root of the issue, but the lag was still there. I also removed all entities from the entire world, which did not help, either.

More details
This issue is not about loaded/unloaded chunks. Let's say chunks A, B and C are right next to each other, and B is an affected chunk. I can freely move around in A. Once I enter chunk B, the framerate drops as described above. Once I leave chunk B and enter chunk C, my framerate goes back to a normal value (after a few seconds). So, chunks that are loaded when I enter B are certainly still loaded when I enter C.

The issue seems to affect physics/time, too: When I stand outside an affected chunk, and throw and enderpearl through the affected chunk and into a non-affected chunk, then the enderpearl will travel with normal speed. However, when I stand inside the affected chunk and throw an enderpearl from there towards a non-affected chunk, then the enderpearl will travel extremely slowly while in the affected chunk.

Also, a player that stands outside of these chunks is not affected by the framerate drop or the CPU load of a player that is inside the chunk. (i.e. I can stand right next to the affected chunk, having normal framerate and CPU load, and watch another player inside the chunk who moves around one inch every 3 seconds.). Mobs behave normal in these chunks, they walk with their regular speed, and track and attack players.

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I will be very grateful for any other suggestions for things I could try. But the fact that 1.7.4 is not affected makes me think that this actually could be a bug related to the new representation of block data.

The four attached screenshots show that the issue does indeed start right at the border of the chunk. If necessary, I can provide a world download for my world.