The Ethereum Blockchain was attacked using a computational Denial-of-Service attack. This affects blocks 2.4 million to 2.7 million.
Today the network was attacked by a transaction spam attack that repeatedly called the EXTCODESIZE opcode, thereby creating blocks that take up to ~20-60 seconds to validate due to the ~50,000 disk fetches needed to process the transaction.
If you're using geth, use the options
--cache 1024 --targetgaslimit 1500000 --gasprice 20000000000, if you're using parity, use the options
--cache-size-db 1024 --gas-floor-target 1500000 --gasprice 20000000000 --gas-cap 1500000, which will increase processing speed.
For more info, see this post on the Ethereum Blog
Alternatively you could use a light sync, which only validates new blocks. Use
geth --light. Now,
geth --fast --cache=1024 should be sufficient as there are more than the 1024 blocks after the affected blocks which are validated on the fast sync.