I found this really helpful article from Rickard Nobel who has done a set of testing showing the performance improves when using VMXNET3 network adapters over the emulated E1000 cards.
http://rickardnobel.se/vmxnet3-vs-e1000e-and-e1000-part-1/
http://rickardnobel.se/vmxnet3-vs-e1000e-and-e1000-part-2/
Check it out and leave a comment.