@RaHu

In theory you would want to change the MTU of parent interface in OPNSense to 9000. In my case testing is not possible since I run all vlans off a since interface (2x10gbe NIC) and do not want to change the MTU of systems outside my virtual environment. Anyways, with the change mentioned above you achieve 10gbe speeds between VMs.

You should be able to go to the network tab for the VM, click the network it's currently connected to, and then change it to the new one. If you have the management tools installed this can be done live, otherwise you'd need to shut the VM down.

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