In a Minecraft world with any more than around 50 piston_extension blocks, the frame rate exponentially drops until it reaches 0-5 FPS. Chunks also stop loading/rendering at this point.

When leaving the area, (unloading the piston_extensions) the frame rate return to normal. When pausing the game in single player (pressing esc), the frame rate return to normal, and all chunks load. Deleting the piston_extension blocks also causes the frame rate to return to normal, and all chunks to load.

This issue happens regardless of whether the world is being run on a multiplayer server, or being opened locally in single player.

The frame rate drop does not happen in 14w32b.