Slow copying from the My Cloud USB port is a well known issue. If you do a forum search you’ll find numerous topics and complaints about it. The workarounds are generally the following.
If you have a large amount of data to transfer it is recommended to connect the USB drive to a computer, connect the computer to the network via Ethernet cable and use Windows Explorer or Mac Finder to transfer the data from the external USB drive to the My Cloud.
If you connect the external USB drive to the USB port on the My Cloud, then do not use Windows Explorer or the Mac Finder to initiate the data transfer/copy. Supposedly what happens if you do is the data flows from the USB drive to the computer and back to the WD My Cloud hard drive, there by giving very slow copying speeds. Even slower speeds if the computer is connected via WiFi rather than Ethernet. The recommendation is to use the WD My Cloud Desktop app (or the WD My Cloud mobile app) to copy folders and files from the USB drive attached to the My Cloud and the My Cloud drive.
Or one could use SSH (enabled via the Dashboard) to initiate the copy of files between the attached USB drive and the My Cloud drive.
More on the various speed issues are discussed at the following thread: https://community.wd.com/t/before-you-pack-up-your-wd-and-return-it-lets-talk-about-copying-speeds/91887
