That's odd. A few things you could check:
- Are you able to plug a physical win 7 host into the same switch and run a speed test?
- Are VMware Tools installed / up to date?
- Is this host under heavy CPU load?
- double check to make sure the portgroup isn't running in promiscuous mode - this would send a lot of unwarranted traffic to each host.
Do you have any other OS VMs running on there (e.g. Server 2012) that you can test from?