what cable are you using atm? Cat5e can handle 1gbit no problem. VM talk out of their arse when it comes to this kind of thing. When they forced a shub1 on me a few years ago my speed tests went from a rock solid 49.xxmbits on my vmng300 (50mbit tier) to 35mbits on the shub (100mbit tier) and all tech support (UK) had to say was that although my pc was showing as being connected at 1gbit to the shub, it might not actually be connected at 1gbit and Windows ins reporting it incorrectly.
As much as I would love to blame this on the shub, I think the issue is with the realtek drivers. I have seen this problem mentioned at least twice before last year and it took a bit of trial ad error to get it fixed. I am pretty sure the peeps ended up having to use slightly older unsigned drivers in order to fix the problem which isn't really the way you want to go.