I recently had this problem, and after getting a new router, and a visit from Comcast, the problem turned out to be a setting, specifically the QOS Packet Scheduler. I have no idea how that setting got turned on, but turned on it was. It reduced my connection from 290 megs per second to 4.5. Quality of Service indeed. All I had to do was uncheck that box and it everything was fine again. I guess Windows 10 uses different settings for the router vs direct connection to the modem.
||Written By Steve French|