Sequential Mixed Data Sweep
Applications like video editing, which pull files from the NAS, involve reading and writing data at the same time. SATA devices are inherently handicapped in these workloads because they communicate over a half-duplex interface. SAS is full-duplex, but most of us can't afford to run arrays of expensive, hot-running SAS-based disks. Mounting an iSCSI share in a desktop allows the host PC to see the resource as a local drive. The NAS can then work around the limitations of SATA with cache.
That works out great when you have sufficient read and write performance to sustain solid mixed workload numbers. Unfortunately, the W5000 doesn't fare particularly well writing data, and in this sequential chart, you can see the metric is mired as a result.