Do you have another gigabit device excluding your cloud device to test transfers from your laptop to rule it out? You need to test every node/link on your network before concluding the cloud device is at fault. Even faulty cables, outdated nic drivers or wrong tweaks could lead to a bad or noisy network environment.
10-12MB/s transfer from your “gigabit ethernet” laptop to your “fast ethernet” pc is quite normal for 100mbps link. But you can’t compare this to transfers within two separate share folders as you already known by now that the data needs to make a round trip, shareA->pc->shareB and yes it goes the same even for USB shares attached to your cloud which why I recommended sftp to perform internal large transfers earlier.
The WDMyCloud manual did actually gave an indirect hint to avoid transferring large data from USB device but to link the USB device to your PC instead. Though they should have given a clearer explaination. Refer to the test I just did below, shares to shares doesn’t really work out well. I would rather ssh to the device and perform mv or cp commands that completes within seconds…
Within two WDMyCloud Shares:
WDMyCloud to Laptop:
Laptop to WDMyCloud: