D-Link DI-704P router

I recently bought a D-Link DI-704P router with the intent of using it as a first line of defense in front of a Linux server I have on a public IP.
The basic stuff is working as expected, mostly “virtual servers” or “port forwarding” which is used to allow access to the select services I want to expose to the internet.
The problem is through-put. I have never seen it do more that about 4 Mbytes/s, and it averages around 3 Mbytes/s, which is a disappointment as I have a line capable of at least 6 Mbytes/s.
Maybe I was too stingy getting a cheap boardband router for a high speed line.