Slow speeds across the USB 3.0 host port on the My Cloud is a well known issue in this subforum. Lots of threads on it. When copying to or from the USB drive attached to or from the My Cloud its best to either use the WD My Cloud Desktop for Windows/Mac, WD My Cloud mobile apps for Android/iOS, the MyCloud.com web portal, or log into the My Cloud via SSH and issue the copy command from the SSH command line. The problem appears to stem from using Windows Explorer (and possibly Mac Finder) and the data flowing through the computer rather than between just the My Cloud and its attached USB drive. Toss in copying over WiFi and it gets even slower. See the following thread(s) that cover this copying issue.
https://community.wd.com/t/slow-usb-3-0-transfer-speeds-one-twentieth-expected-speed/143851
