low kvm internet speed

Hi,

I have a speed problem in my server (os: Unraid) I have a debian vm which acts as a router which has 2 interfaces
-wan 10gb passthrough (wan speed: 8000Mbps)
-lan (virtual network virsh) (all vm have virtio for the network)

<network ipv6='yes'>
  <name>lab-network</name>
  <uuid>9076910b-63df-470f-862a-606414dd750a</uuid>
  <bridge name='virbr1' stp='on' delay='0'/>
  <mac address='52:54:00:a7:8f:4b'/>
  <ip address='192.168.0.1' netmask='255.255.255.0'>
  </ip>
  <route address='192.168.0.0' prefix='24' gateway='192.168.0.10'/>
</network>

iperf between a vm w10 and the vm debian from lan to lan:

But the speed I have with a speedtest :

In your opinion, the clamping comes from where ? After doing the test with iperf I have the impression that the clamping comes from the nat of the lan to the wan because in lan I have a connection of 7Gbits/s

Thanks for your help.