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I am running into severe performance issues using SSH/Rsync on a Synology NAS (model DS413j if anybody knows anything about it).

Both the sshd and the rsync processes are taking 100% CPU on the NAS, and the transfer speed is about 6 or 7MB/s, which is far below the usual network speed -- a Gigabit link that works at more than 50MB/s between other computers on the network. This -- and multiple forum posts about the same kind of issues -- leads me to believe that the bottleneck is the SSH session encryption.

Is there a way to reduce the session key size (or to change to a faster algorithm if necessary)?

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If it's in a local network you can still switch to the non-encrypted equivalents : FTP, telnet, rsh... At least you can test if your assumption about the encryption being the bottleneck is valid. –  Kwaio Aug 23 '13 at 14:30
I have tested that as well, and the performance is fine, but I want to communicate with an external server, and while cryptography strength is not a huge concern, I'd rather not have things go over the wire completely unencrypted. –  F.X. Aug 23 '13 at 14:44
You may want to have a look at "High performance SSH patch" : cyberciti.biz/tips/sshd-server-optimization.html –  Kwaio Aug 23 '13 at 14:54
you can also force the use of the blowfish cipher which is less cpu-intensive : scp -c blowfich ... (don't know how to specify that with rsync over ssh tho) –  Kwaio Aug 23 '13 at 14:56
Remember those NAS have low-speed ARM processors, you may also have reached what your device is capable of... –  Kwaio Aug 23 '13 at 15:05
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