VirtualBox internal network too slow (frequently stalled)

I have two guests run in Windows 7 host, both of them are Debian-based Linux.

• Linux 1
  • eth0: NAT
  • eth1: inteal network (managed by Gnome by using Link-Local in the settings)
• Linux 2
  • eth0: inteal network (manually set up by using ifconfig)

I copied a large file from Linux 2 to Linux 1 by using scp, the speed was not more than 50 KB/s. I tried also copying by using NFS (nfs-keel-server) and also by using nc. However they are all the same, very very slow.

I've confirmed that there is only one hop between the two guests by using traceroute.

Is there any solution to speed it up?