If I create a empty world, the game lags on the server side, which means, that every it takes very long to perform any update (mob movement, redstone). Normal worlds work fine. Also this lag didn't occur in snapshot 14w21b and before.

1. Create a flat world with preset "3;minecraft:air;1;" (empty world, without quotation marks)
2. Try using game elements which require regular updates

If I use a world like this on a server, my chat gets spammed with the "Can't keep up" messages, for example "[xx:xx:xx] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3324ms behind, skipping 66 tick(s)" every few seconds.