Im running Virtualbox on a Ubuntu 12.04 server (host) and I'm running a Windows 7 as guest os. Im using the (virtual) Intel PRO/1000 MT network card.
I get good network performance for download using both nat and bridged network settings but upload speed is really slow using nat.
I have tied this on tow different servers, one brand new, and one a several years old, both gave the same result.
If you can explain this behavior or have ideas of further test I can perform please let me know.