Rsync: use RSH instead of SSH?

I have a now five year old ReadyNAS Duo that I am using for remote backups over the Internet. It is dog slow. Even though it sits on a 50+ megabit Internet connection I typically only manage to get 3-5 megabit even using the old boring unecrypted FTP.

I would like to use Rsync with it to do hard link incremental forever backups (millions of hard links) but Rsync over SSH just absolutely kills any performance at all due to the tiny weak little brain in the Duo.

Is there a way to configure the Rsync server on the Duo to use just the generic unecrypted RSH rather than SSH? I am already using FTP so you can see security is not the concern here.