FOLLOWING UP:
You guys have been so helpful with every question that I have ever asked that I thought I would follow up on this thread just to add what I have learned from my experience with the "WiFi vs. Cable" issue.
1) I bought the wrong adaptor.
This is correct ethernet adaptor. It's made by Belkin, not Apple, but it's fine. I've had Belkin products in the past, and they have never been a problem.
2) The adaptor gets hot to the touch when in use. I hadn't anticipated that.
3) Using the adaptor with 75-ft. of cable resulted in superfast download speeds — approx. 230 Mbps. — according to online speed tests. However, I did not notice any wildly improved functionality on the sites that I normally visit. That said, I did not attempt to stream a movie.
4) Walking around with 75-ft. of ethernet cable behind me is not going to work for me, so I've gone back to using the Airport Extreme base station.
5) I'm glad I did all this because a) it satisfies me that Spectrum is not fooling around with my Internet speed and b) I now have an adaptor and cable if I ever need a backup.
Thanks again.