Slow iSCSI when using 1 specific iSCSI target to Compellent

Hello,

We have an older SC 40 providing 40TB of iSCSI storage to 8 physical servers in a Dell Blade Chassis (M1000e).  When all four iSCSI targets are connected everything appears normal.  But, we can write to a Compellent volume from a local server disk at 10-20MBps (Boo!).  If we disconnect one specific iSCSI target (Controller 2 port 2) the network transfer speed immediately improves to 110-300MBps (Yay!).  MPIO is setup and appears to work normally as well.

Moving data the opposite way can be transferred at 600+MBps when any/all of the iSCSI links are connected.  We do not have software support with Dell and we cannot upgrade the Compellent OS/firmware anymore.

Any ideas how to troubleshoot this? 

I'm not too familiar with setting up the iSCSI network or fault domains... the networking guy (who doesn't know Compellent) said it all looks normal.  There are no errors or warnings on the Compellent or on the physical server that I can see so far.

SC40 Config
Model: CT-SC040
Version: 6.3.10 (for both controllers)
iSCSI ports mentioned below are 10Gbps
Controller 1

• Port 1, IP=10.26.5.49 (VLAN 5) --> Physically connected to Switch A1 TE1/1/3
• Port 2, IP=10.26.6.51 (VLAN 6) --> Physically connected to Switch A2 TE1/1/3

Controller 2

• Port 1, IP=10.26.5.52 (VLAN 5) --> Physically connected to Switch A1 TE1/1/4
• Port 2, IP=10.26.6.50 (VLAN 6) --> Physically connected to Switch A2 TE1/1/4

Fault Domain 1

• 10.26.5.51 (Preferred port = controller port 1)
• 10.26.6.52 (Preferred port = controller port 2)

Blade Chassis
All servers running Windows Server 2016 Datacenter with latest OS updates, and Dell firmwares for NIC and QLogic NIC software.  2 local disks mirrored.  Each blade server has 2 10Gbps NICs to the switches mentioned below with the appropriate VLANs configured.  All blades running Failover Cluster for Hyper-V VM's, currently running about 50+ VMs.
Switch A1 (10Gbps ports)

interface Te1/1/3
description "10_Link_to_Compellent_1-1"
spanning-tree portfast
mtu 9216
switchport access vlan 5
exit
!
interface Te1/1/4
description "10_Link_to_Compellent_2-1"
spanning-tree portfast
mtu 9216
switchport access vlan 5
exit

Switch A2 (10Gbps ports)

interface Te1/1/3
description "10_Link_to_Compellent_1-2"
spanning-tree portfast
mtu 9216
switchport access vlan 6
exit
!
interface Te1/1/4
description "10_Link_to_Compellent_2-2"
spanning-tree portfast
mtu 9216
switchport access vlan 6
exit