I'll start by conceding that the problem I'll be discussing was MINE, not Xfinity's.
But we didn't know that for a long time.
Recently my 2 year contract with Xfinity lapsed and my bill jumped $50 per month. I called to renegotiate.
They responded with a new plan that had the same TV channels and bumped the Internet speed from 200Mbps to 800Mbps. While I didn't NEED that speed increase, faster is always better.
So after a couple of days, I tried a speedtest.
Hmmm. 250Mbps. What's up with that?
So I looked at my modem, a CISCO DPC3008. While it is DOCSIS 3.0, it only has 8 download channels. This limits it to 340Mbps.
Maybe that was the problem. Not.
But it was time for a new modem anyway so I got an Arris SB6190. It was still DOCSIS 3.0 but had 32 download channels for 1.4Gbps.
Maybe that would fix it. Not.
So I called Xfinity for support. I got a representative in Honduras who was very thorough. His thinking was that there was a cap still in place somewhere but he couldn't see it. So he dispatched a technician.
The technician showed up. His diagnosis was that I had a bad coupling on the coax going into the modem. Not.
I was still at 250Mbps.
I placed another service call. This time the technician didn't even show up. He just called.
He said that I needed a different bootfile. His attempts at downloading a new one didn't work. He said that was because I needed a DOCSIS 3.1 modem.
And that's the end of the Xfinity lack of support story. Hours and hours of my time. Several hours of Internet down time while replacing/testing hardware. Hundreds of dollars spent. Two technicians dispatched neither of whom was capable of diagnosing a problem.
So I decided to take the advice I gave to one of my Unix admins when he was troubleshooting a dial-out modem on an HP 9000.
FOLLOW THE WIRE.
I took a laptop with a gigabit Ethernet port and plugged it directly into the Netgear CM2000.
Bingo! I got 650Mbps.
Then I plugged that laptop into the LAN port on my router.
That pointed directly at my Asus RT-AC68R router.
As Pogo said, "We have met the enemy, and he is us."
So off to Google I went.
What I FINALLY found was that the RT-AC68R defaults to using the CPU to perform NAT acceleration. But the RT-AC68R has dedicated hardware that it can use. When I dug down into the settings and switched "NAT Acceleration" to "Auto", all was well!
The download speed jumped to 950Mbps!
The switch point where you should use the dedicated hardware is 150-200Mbps so I hadn't stumbled on it earlier.
Then I switched back to the Arris SB6190 and returned the Netgear CM2000. I still got 850Mbps.
Lessons learned: 1) Fast home Internet is a challenge and 2) Xfinity is no help.