On the issue of copying speed being slow when transferring files between a USB drive connected to the My Cloud and a Share on the My Cloud. Its a well known complaint with many threads discussing it.
If your going to move files from the USB drive attached to the My Cloud the better way (and generally faster way) is to use the WD My Cloud Desktop program to move the files. Or use the WD My Cloud mobile app for Android or iOS to copy the files. Or use SSH to connect to the My Cloud (after activating the SSH option via the My Cloud Dashboard) and issue a copy command to copy files from the attached USB drive.
The reason why the copy is slow when using Windows File Explorer or Mac Finder when copying files from a USB drive attached to the My Cloud to a Share on the My Cloud is the data apparently is flowing from the USB drive through the My Cloud, to your computer then it flows back to the My Cloud to the Share folder on the My Cloud. If your attempting to trigger this copy via WiFi it will further result in slower speeds when using Explorer or Finder.
The other option when transferring a large amount of files from a USB drive to the My Cloud is to connect the USB drive to the computer, connect the computer via Ethernet cable to the same router/gateway the My Cloud is connected to, then use Windows File Explorer or Mac Finder to copy the files.
There is much more on copying speeds at the following thread.