Why is a wired connection to xb8 sooo much slower than Wi-Fi?

I am trying to diagnose why my wired connection to this new router is so terrible. Ex) while sitting next to it I easily get with-fi speeds right around 1g. If i plug a cable into the 2.5G port my speed drops to 150-200mbps. This is using a new MacBook Pro with 10G NIC, and multiple different cat6 cables. After many frustrating hours trying to negotiate the CS morass at Xfinity, I finally decided to get new hardware. Traded in my xb8 for a new one, but the same [Edited: "Language"] is still happening, with the added bonus that the phones don’t work anymore. I was able to set up a service call at the prompting of the agent. About an hour later I got a call from “advanced” tech support, whose main job was apparently to get me to cancel the service call. We went through the same troubleshooting steps I had been through about 100 times earlier. Finally, he had me try to plug into a different port and, what do you know, 1G speed test. I thought I was good, and he canceled the service call. Unfortunately that was the only fast speed test, now back to 150-200 wired. So, how is wi-fi faster than direct wire? Why are ports all acting differently? Is this just a [Edited: "Language"] modem?