The Ethereum network is currently undergoing a DoS attack

URGENT ALL MINERS: The network is under attack. The attack is a computational DDoS, ie. miners and nodes need to spend a very long time processing some blocks. This is due to the EXTCODESIZE opcode, which has a fairly low gasprice but which requires nodes to read state information from disk; the attack transactions are calling this opcode roughly 50,000 times per block. The consequence of this is that the network is greatly slowing down, but there is NO consensus failure or memory overload. We have currently identified several routes for a more sustainable medium-term fix and have developers working on implementation.

Source:
https://blog.ethereum.org/2016/09/22/ethereum-network-currently-undergoing-dos-attack/