And that has to be done through an ssh tunnel? If not you could use and FTP serrver and wget. That is without doubt the fastest solution.
If you need a tunnel (for transfers over internet, for instance) you can also use the tunnel option of ssh:
tar cz /path/to/source | ssh user@<other-nas-ip> "dd | tar -C /path/to/target xz"
Preferably with a login-less connection. This way your data is compressed before it's encrypted, and because gzip compression in most cases is cheaper than ssh encryption, this will give a higher throughput/lower cpu load.