VM virtio network performances

Hi all, I am using libvirt for more than 10 years, and I have installed it on lot of servers, with dozens of VMs. Network performances from VMs to host have always been at top. But first time I am in front of this problem: Replacing an old platform made of 2 servers Fedora (36 upgraded to) 37 by 2 servers (H/W stronger) with Fedora 39, same .qcow2 diskimages for VMs, I found out that response times are 2,5 times higher on the new platform than the old one. I have check H/W, new are better. I have check standard network configuration, identical, virtio is best (same as older) like I saw in my google searches. Then I checked network performances: On old platform, from VM1 to guest: iperf3 [ ID] Interval           Transfer     Bitrate         Retr [  5]   0.00-10.00  sec  27.7 GBytes  23.8 Gbits/sec    0             sender [  5]   0.00-10.00  sec  27.7 GBytes  23.8 Gbits/sec                  receiver On new platform: [ ID] Interval           Transfer     Bitrate         Retr [  5]   0.00-10.00  sec  2.67 GBytes  2.29 Gbits/sec   17             sender [  5]   0.00-10.00  sec  2.67 GBytes  2.29 Gbits/sec                  receiver So intrinsic network performances are ten times better on old platform. All VMs network definitions are identical: <interface type='network'>      <mac address='52:54:xx:yy:zz:53'/>      <source network='default'/>      <model type='virtio'/>      <address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/>    </interface> On the host: <network>  <name>default</name>  <uuid>cfcfeeb6-3aa2-4732-9ca4-caf5b6554f2a</uuid>  <forward mode='nat'/>  <bridge name='virbr0' stp='on' delay='0'/>  <mac address='52:54:00:82:33:0c'/>  <ip address='10.0.10.1' netmask='255.255.255.0'>    <dhcp>        <range start='10.0.10.170' end='10.0.10.195'/>    </dhcp>  </ip> </network> chatGPT has no clue too. May I have some help? Thx Patrick