I came about that my SSH setup was only able to transfer ~12MBytes/s. After some digging I found out that SSHv2 is by default using "3des" as the cipher.
When I set it specifically with "-c blowfish" to a another secure, but much faster block cipher, I got ~24MBytes/s. If you want you can configure this as the default in your ~/.ssh/ssh_config as
cipher_spec blowfish