The Ethereum network is currently suffering from a DoS attack.

Urgent All Miners: Your network is under attack. The attack is a computational DDoS. Miners and nodes have to spend a very long time processing some blocks. This is due to the EXTCODESIZE opcode, which requires the node to read state information from disk, although the gas price is significantly lower. The attack transaction calls this opcode approximately 50,000 times per block. This results in significant network slowdowns but no consensus failures or memory overloads. We have now identified several pathways for more sustainable medium-term fixes, and developers are working to implement them.

Consider switching to parity mining. Use the following settings:

–cache size-db 1024 –gas floor target 1000000 –gas price 50000000000

Parity Mining Guide:

https://docs.google.com/document/d/1j3v8bKjXQcMNNdx5Juwo3PRRCcHy8XcA6R9-0W7NCso/edit

https://github.com/ethcore/parity/wiki/Mining
To continue using geth, use the following settings:

–cache 1024 –targetgaslimit 1000000 –gasprice 50000000000