just hit the same issue...
every network connection (transfer) it starts 50-60 Mbps and than it goes down to 11Mbps and fluctuates there.
it doesn't matter 1,2,3 nics / Bond / e1000 or vmxnet3 -> transfer between 2 machines stays ~11Mbps .
*** if i open a new transfer from another machine i get the same results, but i now have 2 transfers that have ~11Mbps EACH.
*** if i attach another drive , transfer between raid volume and drive it's working ok - 50-60 MBps .
it seems to be an issue in NIC management ?
is there a vmwaretools package for DS3617xs & DSM 6.1.3-15152 ?