I recently had 1Gbps fiber ethernet installed and I get that speed when my computer is attached directly to the cable company's box. But when I plug my computer into my router (R7000 Nighthawk) it drops down to 200Mbps.
I have up-to-date software and firmware. I reset the router to factory settings etc and no change.
My connection is ethernet cable (cat5), not wireless. So there should be no slowdown when going through the router.
after the reset, make sure not to reinstall using a backup. Also, make sure not to enable qos, access control, traffic monitoring, or parental controls. If you enabled any of those, factory reset again. After its reset, test it in its default configuration.
