Hopefully you would actually be able to get a full speed transfer between the Ethernet ports; I had to go back to the factory firmware on my router in order to take full advantage of my 1 gigabit Internet connection.
If it was several years ago and on a router that was not too expensive, then there's a good chance it's CPU was still an ancient MIPS core or two, mated to vastly more modern radios. Cheap routers have only recently started moving to ARM cores with enough performance to actually do routing, traffic shaping, etc. at gigabit speeds. There were a ton of routers sold that could only get close to a gigabit of WAN to LAN throughput by offloading NAT to their built-in Ethernet switch.
