Oracle VM VirtualBox provides a variety of virtual network adapters that can be attached to the host's network in a number of ways. Depending on which types of adapters and attachments are used the network performance will be different. Performance-wise the virtio network adapter is preferable over Intel PRO/1000 emulated adapters, which are preferred over the PCNet family of adapters. Both virtio and Intel PRO/1000 adapters enjoy the benefit of segmentation and checksum offloading. Segmentation offloading is essential for high performance as it allows for less context switches, dramatically increasing the sizes of packets that cross the VM/host boundary.
Neither virtio nor Intel PRO/1000 drivers for Windows XP support segmentation offloading. Therefore Windows XP guests never reach the same transmission rates as other guest types. Refer to MS Knowledge base article 842264 for additional information.
Three attachment types: Internal, Bridged, and Host-Only, have nearly identical performance. The Internal type is a little bit faster and uses less CPU cycles as the packets never reach the host's network stack. The NAT attachment type is the slowest and most secure of all attachment types, as it provides network address translation. The generic driver attachment is special and cannot be considered as an alternative to other attachment types.
The number of CPUs assigned to VM does not improve network performance and in some cases may hurt it due to increased concurrency in the guest.
Here is a short summary of things to check in order to improve network performance:
Whenever possible use the virtio network adapter. Otherwise, use one of the Intel PRO/1000 adapters.
Use a Bridged attachment instead of NAT.
Make sure segmentation offloading is enabled in the guest OS. Usually it will be enabled by default. You can check and modify offloading settings using the ethtool command on Linux guests.
Perform a full detailed analysis of network traffic on the VM's network adaptor using a third party tool such as Wireshark. To do this, a promiscuous mode policy needs to be used on the VM's network adaptor. Use of this mode is only possible on the following network types: NAT Network, Bridged Adapter, Internal Network, and Host-Only Adapter.
To setup a promiscuous mode policy, either select from the drop down list located in the Network Settings dialog for the network adaptor or use the command line tool VBoxManage. See Section 7.8, “VBoxManage modifyvm”.
Promiscuous mode policies are as follows:
deny, which hides any traffic not intended for the VM's network adaptor. This is the default setting.allow-vms, which hides all host traffic from the VM's network adaptor, but allows it to see traffic from and to other VMs.allow-all, which removes all restrictions. The VM's network adaptor sees all traffic.