Very slow wired speed, there must be an answer

I recently upgraded to the XB8 modem from the XB7.  I have 1.2Gb service, one two-way splitter before the modem.  All gateway upstream/downstream stats are clean and within expected tolerances, no uncorrrectable words.  Doing an Xfinity speed test to the gateway shows 1200-1400 Mbps.

With the XB7, I regularly got 300-500Mbs on 5GHz wi-fi on most devices and 800-900Mbps on wired connections.  On the XB8, I'm lucky to get 200Mpbs on 5GHz wifi, and consistently below 100Mbps on wired devices.  The only device that gets good performance is my phone when connected to 6 GHz wifi, which averages 900 Mbps.  I replaced the gateway, without any improvement.

I have two Microsoft Surface, Windows 10 laptops I have tested with the same results.  I also have two ethernet adapters that connect to the laptops in different ways (USB and a dock connector).  I have also tested both of these laptops and network adapters at my second Xfinity location, which has 200Mb service and an older surfboard Xfinity modem, and I get full, expected service speed on both laptops on both wifi and wired, and it's faster than the XB8.

I have tested with the splitter removed so the gateway is the only device connected, with no change.

I have a Moca adapter on my second coax run off the splitter, and with both laptops, with either network adapter, I am able to average 700-800 Mbps, which seems to point to the fact that it's not the laptops or network adapters.  

When running the speed test, the behavior makes it appear that the wired connection is being negotiated to 100 Mb as opposed to 1Gb, even though the gateway and Windows show it's connected at 1Gb.  To support that suspicion, when I first boot the laptop, if I run a speed test as quickly as I'm able, I can get one test at approx. 900 Mbps, and then subsequent test show the speed dropping to below 100Mbps, where it ultimately stays.  I do not have bandwidth limiting enabled in Windows.

Right now, I'm thinking of buying a second Moca adapter just to get something closer to the service I pay for, but I shouldn't have to do that.  I am hoping to find someone that's been through this before and has the magic bullet to fix this.  I've exhausted every possible solution I've come across, from swapping XB8 gateways to reseting network adapters to changing advanced network adapter and Windows settings.  The common denominator seems to be the XB8 gateway.  Once I installed it, my network performance has never been the same.

Thoughts?