I have tested how fast are the writes on the mounted samba drive.
I think we have found your problem, your network is too slow.
Bad wire, bad switch, bad router, bad network card, ...
Too long wires, too much traffic, ...
wdmycloud 3.10.70+ #5 SMP Wed Jun 7 23:38:41 +05 2017 armv7l Linux
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-120.00 sec 13.0 GBytes 930 Mbits/sec 765 sender
[ 4] 0.00-120.00 sec 13.0 GBytes 930 Mbits/sec receiver
You should get arround 1000 MBits.
Connect the WD My Cloud without any switch/router to your pc and test.
When you get better performance with iperf3, than your samba will be faster too.