I recently purchased a NAS and it offers encryption. It is a QNAP TS459 Pro II.
How much will performance be impacted if I choose to encrypt the data on the device?
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It depends on the workload. It is better to do a load test for your workload. Encryption increase the access time for the data and it might lower the bandwidth too. So loads that have numerous small requests will be more affected. If you have large files that are uploaded or downloaded at once (e.g. no database running over NFS), then you might not notice any difference.
The QNAP NAS devices are but a Linux box with a web interface. Apparently, performance will suffer greatly - this post at the QNAP forum describes a throughput drop from 60 MB/s down to 16 MB/s. The CPUs are not too powerful and since dm-crypt is used for encryption and it does not support using more than one thread, it won't even use more than one core of the 2-core Atom CPU (at least if not more than one device is set up for encryption).
Another route to go would be using self-encrypting drives - this kind of hardware would ensure encryption and data-at-rest-protection without any performance hit. Someone appears to have got it working with the QNAP by using the shell and editing the config/script files.