Depending on the hardware involved, you can dramatically speed up an ssh pipe by changing the encryption type, or turning off compression.

For example, if you're trying to clone a disk from a machine with a weak cpu, and that disk contains mostly media files (non-compressible avi's, mp3's or jpeg's) then trying to compress the stream will be a waste of cpu time. Also, if you're using a strong encryption scheme, then that is also more taxing on the cpu.

So, if security isn't the highest priority then you can potentially speed up a slow disk clone quite easily.

My sender was an old machine so I wanted to clone the disk to a new faster machine.

# my original .ssh/config contained:
Compression yes

# naive/default sender
dd if=/dev/sdb | ssh other_host dd of=/dev/sdb

This was ridiculously slow, I was getting roughly 10MB/s on a gigabit network.

# "smart" sender
dd if=/dev/sdb | ssh -o Compression=no -o Cipher=arcfour other_host dd of=/dev/sdb

This brought the transfer up to 60MB/s, the speed of the hard drive on the slow machine. arcfour is a weak but simple (ie: fast) encryption cipher.

To make this permanent, you can change your .ssh/config to include:

# my new .ssh/config contains
Compression no
Ciphers arcfour,aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc

Or if you don't need the fancier abilities of ssh (multiple hops, encryption, etc) then just use netcat.

# receiver
nc -l 12345 | dd of=/dev/sdb

# sender
dd if=/dev/sdb | nc other_host 12345

But this is a bit more annoying as you have to manually run a command on the other_host and you have to make sure that of=something of the receiver matches if=something of the sender.