This is a bug Etho pointed out in a recent Mindcrack LP.

To reproduce, create a Redstone Ready superflat world, face south, and run

/fill ~ ~ ~2 ~40 ~40 ~2 stone

/fill ~ ~ ~1 ~40 ~40 ~1 wall_sign

Now you have a big wall full of signs. Walk around to the back of the wall, so you cannot see the signs. Look at the wall and wiggle your mouse a little. FPS goes down a lot, compared to a similar experiment without the signs.

In practice, this means that a 'spawn village' with many signs will lower FPS for all players, even when signs are hidden underground or inside houses.

This does not seem to affect every tile entity. For example, change wall_sign to command_block and I don't see an FPS drop.