I first saw this related to newer WD Red drives, and it appears Seagate is doing the same.
Shingled Magnetic Recording (SMR) layers data when written to tracks on the drive, improving data density, but slowing the write process. This would be fine for, say, a DVR that writes once and reads multiple times. For a NAS, it means that a change in the data for a track requires rewriting the track multiple times to layer the data, thus killing throughput.
For WD drives, the telltale part numbers have EFAX in them, whereas non-SMR drives have EFRX in their part numbers. WD SMR drives also have 256MB of cache to support the multiple rewrite of the tracks.