I'm only receiving 500mb down on the Gig Tier plan. Upload is 40mb which is fine. I have the Arris SB8200, a docsis 3.1 modem recommended by Comcast. Speed tests were done using speedtest.net and xfinity speed test while connected directly to the modem via ethernet. Both yeild similar results. Help?
I'm going to guess that the signals are fine with 500x40. I'm guessing the modem needs a hard reset and/or the Ethernet cable may be an issue. The other thing that comes to mind is the Ethernet NIC in the PC is set to 1/2 duplex. For the WIFI though the base NIC usually only supports about 500M throughput because the OEM's are cheap. Upgrading the card will net more speed and the router could be a potential bottleneck though as well but, since it was bypassed for the test it's ruled out at this point.
The NIC was set to full duplex. I should mention 500 is max I ever see. Usually around 300 - 400. This has been since it was installed last week. Hard reset didn't produce different results. I've also tried several ethernet cables. Also have tried multiple computers/laptops connected directly to the modem. The built in speed test in my Ubiquiti Unifi router also produces around 500 max download.
Signals look fine. Everything you're saying should get you over 500M but, for whatever reason the modem's just not doing it. AFAIK CC doesn't offer a 500M tier so it's kind of an odd number to be at for speeds.
The only other thing that comes to mind is I have the MB8600 same guts as your modem but slightly different. When I dropped my DS power levels down to the 2-4 db range I was getting 25% less speed than when I bumped it up to 10+ DB by removing all splitters from the mix and was able to get 950+/45 wired. I would take it to the source and test from there and if it's still bottomig out at 50% then it might just be the modem.