A residential client I have has found (in Lower Alabama) when using only the Mediacom modem to the his PC with an dynamic WAN IP the speeds are as they should be. When he places a router/firewall on the LAN side everything slows way down. He should have 11 MB down 1.5 MB up. With the router in place he is getting about 2.15 MB down and .009 - .51 MB up.
We have tried two different routers, both lower end. The first thing I did was have him purchase a Ethernet only router and branch the wireless router from there. No difference. I use COX and I had some issues but when I replaced the DOCSIS 1.0 modem issued by COX with a DOCSIS 2.0 - everything worked fine. But my troubles were only getting 10 -11 MB down instead of 28 MB down. Upstream was OK. Now I run as advertised by COX..