As you’ve discovered, it’s not the MTU that’s at issue. It’s the segment size defined in SMB and TCP stacks which are different.
The My Cloud gets its higher performance due to large TCP / Samba window sizes – but if these large transmissions overwhelm the switch’s buffers, packets are going to get lost which yield retransmissions and a slowdown in performance.
Read more here:
http://community.wd.com/t5/WD-My-Cloud/HOW-TO-Troubleshoot-NAS-Performance-concerns/m-p/618695#M1256
