Because you had multiple physical independent servers before, and now you have all of those same network interfaces cramming through one virtualized interface.

1. Are you running these interfaces in bridge mode?
2. What’s the link speed of the virtual interfaces?
3. What’s the link speed of the physical interface on the machine they’re all hosted on now?
4. Do you happen to have any QoS or other traffic steering happening on your router right now? It’s certainly going to get confused when you have all these VMs vying for traffic from the same physical interface as your router sees it.