Slow Ethernet Connection between XB8, Mac mini, and Netgear Orbi Router

I’m having an issue with a wired ethernet connection.  Here’s the background…

I have a Mac Mini M1 with a 1 Gbps ethernet port, a Netgear Orbi RBR850 router, and an Xfinity XB8 gateway with (3) 1Gbps ethernet ports and (1) 2.5 Gbps ethernet port.  The router is in bridge mode with a 1.2 Gbps internet plan.  I’m running Cat-6 cables throughout, none longer than 6 feet.  All devices are updated to the current OS and firmware.

The problem is that, when the router is cabled into the 2.5 Gbps port on the gateway, the measured speed to the Mac’s ethernet port is in the 350 Mbps range.  Nicely enough, the WiFi speeds to the Mac still run in the 850 Mbps to Gigabit range.  Dashboards indicate that the wired connection between Mac and router is 1 Gbps.

Lots of other things are working right…WiFi to all my other devices (mostly Apple) is fine.  When I plug the router into one of the Gigabit ports on the gateway, I understandably get near-Gigabit WiFi to all devices and a Gigabit ethernet connection all the way to the Mac.  When I bypass the router and plug the Mac straight into the 2.5 Gbps port on the gateway, I also get the expected Gigabit Ethernet connection.

The wired connection to the Mac worked fine on the Xfinity gateway that I had just prior to the XB8.  I can live with this, given the strong WiFi to the Mac, and perhaps a future update will fix things.  At the same time, it’s leaving me a little perplexed.  I’ve read many stories like mine but have found no solutions.

I’m told that the 2.5 Gbps port on the XB8 uses IPv6, and have tried both the “autoconfig” and “Use DHCP Server” settings in the IPv6/LAN Setup section on the router without joy.

Thoughts are welcome and thanks for listening…