Virtualbox limits bandwidth

[Mavin]

Hi

After installing Virtualbox on my Vista Ultimate 64Bit my nic (Marvell 1Gbps) only measures 47Mbps download when doing speed tests against a test site. Before it was 86Mbps whitch is normal. Why is it like that?
I mean i have 1Gbps nic connection to the router so it should not be a physical limit, more a rather logical "reservation limit". this also happends with VM Ware server.
I also have several nics so is it possible to just reduce the speed for 1 nic and use the other for bridging?

Best Regards
Mavin