Upload speed capped at 554 Mbps on one VLAN behind FortiGate, while another VLAN reaches 2 Gbps — only the policy differs

Hello,

I have a 3 Gbps symmetric internet connection. Topology: VM → vSwitch → backbone switch → FortiGate → IPS router → ISP.

Speedtest results from two different VMs:

• 192.168.0.0/24 segment VM: Download 2500 Mbps, Upload 1995 Mbps ✓
• 10.100.10.0/24 segment VM: Download 2421 Mbps, Upload 554 Mbps ✗

Both VMs run on the same hypervisor and both use vmxnet3 vNICs. Download is at full speed on both; the problem is only on the upload direction of 10.100.10.0/24

There are no traffic shaping rules on the FortiGate.

2 gb

        set srcintf "Zone_1"
        set dstintf "net”
        set action accept
        set srcaddr "all"
        set dstaddr "all"
        set schedule "always"
        set service "ALL"
        set utm-status enable
        set inspection-mode proxy
        set profile-protocol-options "custom-default"
        set ssl-ssh-profile "__upg_certificate-inspection"
        set logtraffic all
        set nat enable
 

500mb

        set srcintf "Zone_2"
        set dstintf "net"
        set action accept
        set srcaddr "all"
        set dstaddr "all"
        set schedule "always"
        set service "ALL"
        set utm-status enable
        set inspection-mode proxy
        set profile-protocol-options "custom-default"
        set ssl-ssh-profile "no-inspection"
        set logtraffic all
        set nat enable