After doing some power benchmarking recently (1.2ghz kirkwood with gb net up = 5watts) I decided to work out how fast these arms can do useful work^TM. In the running is a Synology ds212j, DreamPlug running the Freedombox, Nokia n9 mobile phone, and an Intel 2600k just for working out where those relative numbers sit when compared to a desktop machine.
The above image shows the cipher performance of "openssl speed" across many machines. The 2600k is only single threaded, so could be many times faster in real world use by taking advantage of the cores. One interesting point right off the bat is that the 1.2Ghz kirkwood in the synology NAS is bested by the 1.0ghz CPU of the Nokia n9. Arms is not arms.
The RSA sign and verify graphs both show the same large slump for the ds212j unit. So for connection heavy workloads there would seem to be a large difference to the throughput you might get for the ARM you choose. On the other hand, the Dreamplug and ds212j both have similar performance on steam ciphers. So if connections are longer term then the difference will be less.
I would love to add benchmarks for the CuBox and QNAP 2ghz (TS-219PII) NAS units. It would also be interesting to use after market software on the ds212j and see the difference.