More Benchmark Results Can Be Found In the Picture Gallery.
All the NAS servers deliver solid performance in the data transfer rate benchmarks when configured with standard, non-encrypted partitions/folders. The units from Qnap and Synology even achieve data transfer rates of more than 100 MB/s when recording a video file.
It is rather remarkable that, while the performance is almost equal with encryption enabled, the difference is immense with it gets turned off, particularly when you compare the devices from Synology and Qnap.
Overall, the performance hit attributable to encrypting your network-attached storage device is massive, leaving just about a fifth of the normal data transfer rates. And, it makes no difference whether the encryption is at a partition or at a file level.
The same thing can be seen during the “HD Video Playback” test. While the data transfer rates differ when the drives are not encrypted, the performance is approximately the same when it's turned on.
When transferring small files from the NAS, the difference is not quite that huge. But still, performance drops of 50% or more are to be expected when enabling encryption.