Bukkit

Changes:
+ Added automatically updating signs
+ Added config section: update-signsWith a large server the update-signs setting can become laggy. If you experience this, please turn this off.

Why is it so laggy?
What the code does is search for a join sign. But it doesn't know where it is. It basically goes through all the worlds all the loaded chunks and all the tile entity's. If it founds the right sign it'll update.

When are chunks loaded?
Chunks are getting loaded if there are players around them. So the every shunk you can see is loaded and that for each player. How many chunks are loaded can be changed with the render chunks setting in minecraft

What are tile entity's?
A tile entity is not a block, nor a entity. A tile entity looks like a block but has special features that normal blocks don't have. Tile entity's are:

Signs, Banners, Chests, Trapped Chest, Dispensers, Furnaces, Brewing Stands, Hoppers, Droppers, Beacons, Monster Spawners, Note Blocks, Blocks that are getting moved by a piston (included blocks that are being moved with slimeblocks), Piston heads, Jukeboxes, Enchanting Tables, End Portals, Ender Chest, Mob Heads, Command Blocks, Nether Reactor Core (pocket edition), Daylight Sensor, Flower Pots and Redstone Comparators