ho trovato questo nelle Faq di Virtual Box.. puņ c'entrare qualcosa?
9.10.3. Tuning TCP/IP Buffers for NAT
The Oracle VM VirtualBox NAT stack performance is often determined by its interaction with the host's TCP/IP stack and the size of several buffers, SO_RCVBUF and SO_SNDBUF. For certain setups users might want to adjust the buffer size for a better performance. This can by achieved using the following commands, where values are in kilobytes and can range from 8 to 1024:
VBoxManage modifyvm "VM name" --natsettings1 16000,128,128,0,0
This example illustrates tuning the NAT settings. The first parameter is the MTU, then the size of the socket's send buffer and the size of the socket's receive buffer, the initial size of the TCP send window, and lastly the initial size of the TCP receive window. Note that specifying zero means fallback to the default value.
Each of these buffers has a default size of 64KB and default MTU is 1500.