I have a new VMWare ESXi install with two CentOS VMs. VMWare tools installed, no packet loss when pinging. The downstream throughput from the guests to the rest of the true network or the outside world is extremely limited. At some points, it'll drop as low as 10K/s. However, I can transfer between the two guest VMs at high speeds. Also, I plugged a laptop into the same port with the same net config and it doesn't have issues.
As I said, this is downstream throughput only. I can upload files to external hosts at very high speeds.