You have some throttling somewhere.. I suspect one or more interfaces on the ESX host isn't a 1G, it looks like a Fast Ethernet..
I would trace ALL network connections from ESX host, verify those NICs in ESX are showing 1G (or higher) connection, and that port on the actual switch is in fact a 1G or higher speed.
This appears to be a problem with your network setup, and not a problem with ESX.
We get good throughput, so it's difinately NOT ESX or VMware related.
I am sure VMware will tell you the same thing.