Virtualbox, slow upload speed using nat

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http://serverfault.com – 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. (HowTos)