Seems others have various slow copy issues with that unit:
https://community.wd.com/t/n900-central-very-slow-transfer-rate/89197
https://community.wd.com/t/n900-central-slow-internet-download-speed/89499
As we’ve seen with other routers NAS drive performance is slow, lacklustrely plodding along at around 10MB/s via wireless and 20MB/s via LAN.