Make sure that the virtual machine has "paravirtualization interface: kvm" setting for linux guests, that the guest tools are installed and if you really use ne

Make sure that the virtual machine has "paravirtualization interface: kvm" setting for linux guests, that the guest tools are installed and if you really use network, adapter type: virtio-net doesn't hurt. And also, that you have enough RAM. Then, virtual machines in VirtualBox are fine.