that honestly sounds like bad hardware/bad 40g stack card, never seen anything like that before. Can you post your config?
And you're sure it's not an issue with the vlan handling on the end devices, right?
Definitely not an issue of vlan handling on the end devices as I've checked them before with the exact same configuration on my Cisco 3750E with no throughput issues - furthermore I just rechecked the config with ports 1/2/2 through 1/2/5 and 1/2/7 through 1/2/10 untagged on a vlan and had no issues achieving full 10g throughput.
Config:
Current configuration:
!
ver 08.0.20T7f3
!
stack unit 1
module 1 icx6610-24p-poe-port-management-module
module 2 icx6610-qsfp-10-port-160g-module
module 3 icx6610-8-port-10g-dual-mode-module
!
global-stp
!
!
!
vlan 1 name DEFAULT-VLAN by port
spanning-tree
!
vlan 10 name Management by port
tagged ethe 1/1/24
untagged ethe 1/1/1 to 1/1/6
spanning-tree
!
vlan 20 name Hosts by port
tagged ethe 1/1/24
untagged ethe 1/2/2 to 1/2/5 ethe 1/2/7 to 1/2/10
spanning-tree
!
!
!
!
!
ip dhcp-client disable
!
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!
end
VLANs:
PORT-VLAN 1, Name DEFAULT-VLAN, Priority level0, Spanning tree On
Untagged Ports: (U1/M1) 7 8 9 10 11 12 13 14 15 16 17 18
Untagged Ports: (U1/M1) 19 20 21 22 23
Untagged Ports: (U1/M2) 1 6
Untagged Ports: (U1/M3) 1 2 3 4 5 6 7 8
Tagged Ports: None
Uplink Ports: None
DualMode Ports: None
Mac-Vlan Ports: None
Monitoring: Disabled
PORT-VLAN 10, Name Management, Priority level0, Spanning tree On
Untagged Ports: (U1/M1) 1 2 3 4 5 6
Tagged Ports: (U1/M1) 24
Uplink Ports: None
DualMode Ports: None
Mac-Vlan Ports: None
Monitoring: Disabled
PORT-VLAN 20, Name Hosts, Priority level0, Spanning tree On
Untagged Ports: (U1/M2) 2 3 4 5 7 8 9 10
Tagged Ports: (U1/M1) 24
Uplink Ports: None
DualMode Ports: None
Mac-Vlan Ports: None
Monitoring: Disabled