Minecraft runs very slow. Like each tick it 2 seconds, minecarts lag. I have experienced this effect when i was doing stuff with thousands of command block minecarts setting blocks everywhere. Switching back to 14w21b puts everything back to normal.
Environment
Java 64bit, 4G Ram (3.8Avalable), Nothing else open than minecraft
Ticket resolved as incomplete, because no answer in a reasonable amount of time (1+ year), if it still happens, please update the ticket.