To verify that Flow Control is enabled on your physical network switches:
Check each host in your environment. Click Configuration > Networking to determine each iSCSI port group that you have configured.
For each vmnic network card connected to these port groups, trace the network cables back to the port on the physical network switch.
You may also be able to identify the port either by using CDP (blue icon next to the network cards in vSphere client) if CDP is enabled.
Your physical switch may show the MAC/Ethernet addresses of the network cards connected to each port. The Network Adapters section on the vSphere client shows the MAC address of each vmnic.
For each port on the physical network switch that the vmnic is connected to, ensure that Flow Control is enabled.
If Flow Control is enabled on all ports on the physical switch and the problem of high latency remains, open a case with both Dell and VMware Support to continue troubleshooting this issue.