I think I’ve found where the problem is. The fact that I have poor performance also from inside the ZS host make me think that the problem is inside the vm configuration.
Right now ZS is using the rtl8139 driver (kvm/libvirt) for the 3 nic. I’ve created another vm using virtio driver and attached to the provider’s router and I see no problems at all.
I’ll try changing the driver for the ZS host too and I’ll confirm here if it works.