Its because you got conned into buying a cat7 cable and those only work on 10g interfaces......
Not true but cat7 is a waste of money when you are talking about running internet.
Wired should always be the same or faster. I would first hook to the modem to be sure it is not something strange with the router.
It is most likely the ethernet drivers on your motherboard. You need to use the ones directly from the manufacture you do not want to use the ones from microsoft or any other source. If this board can run the ones from "killer networks" I would try to load the normal ones instead. You see lots of reports of issues with these third party drivers.