You should be able to see the cpu utilization in that router. I forget exactly where because there are so many revisions of dd-wrt. If nothing else you can get it with cmd lines.
The throughput of a router is pretty much limited by cpu. It seems you have turned off the feature like qos,firewall vpn etc and it makes no difference. That pretty much leave the NAT function eating the cpu.
This is a rather old router so it has dropped off the testing sites that would show expect maximum throughput using wired connections. 5 or 6 years ago a couple hundred megabit rates was what your average priced consumer routers were getting. The new ones you see now even mid priced device can get over 700 or 800m.
I am going to bet the router is just too old to run that fast.
I suppose you could try a newer dd-wrt firmware and see if you get lucky and it runs faster.
Thanks for the response. You nailed it I think. I ran the test and looked at the CPU utilization while it was running and it was at 100%. Thanks for clearing this up for me!