There is no compatibility issue between a DPC3010 and a router because there CANT be a compatibility issue. Its a transparent switch, so that would be like a dumb switch not being compatible with a router. Also, any compatibility issue would be at layer 1-2 on the OSI model, another words, it would either work 100% or 0%. You wouldn't see a performance issue.
My guess would be something connecting to your network (which would effect both routers) is taking up bandwidth. Maybe try turning everything else off except the PC you had connected direct to modem. If still a issue, it could be wireless interference. To test for that, compare a wireless connection to the router vs a wired connection to the router. That will show you how much your losing between Modem > Router and how much your losing between Router > PC.