Some more hints to improve performance.

It probably makes sense trying to disable rsync compression. Depending on the compressibility of your data, it may be much more favorable to run without any compression at all.

Also as rsync is running over ssh in your case, faster cipher will definitely improve performance, it has been already mentioned in this thread with benchmarks posted. In order to use faster cypher, you can edit '/etc/ssh/sshd_config' on the device and append 'Ciphers blowfish-cbc' or 'Ciphers arcfour' (without quotes) line to the end of it. Rebooting the device may be needed for this change to take effect. Using 'arcfour' is the fastest method, but it is considered insecure and some ssh clients may reject connecting to your device after this change. In any case, messing with sshd config may result in nonworking ssh, so be prepared to restore this configuration file from xterm on the device itself in the worst case.

As for the further optimizations, we may consider either adding assembly optimizations to blowfish cipher, or still try find some way to use OMAP hardware acceleration for DES/3DES/AES.