After seeing that my wireless speeds were much faster than the speeds I was getting over Ethernet, I decided to invest in some new cables. I didn’t know it before, but I saw while I was changing them out that my current cables were Cat 5e. While putting my network together, I had just been grabbing whatever cables I could find in my scrap drawers. Now I have Cat 8 cables and my speeds jumped from 7MB/s to an average of over 40MB/s. It’s a much bigger improvement than I expected, especially for such a small investment.
I’d be interested to see if you swapped the cables back if your local interface negotiated to FE instead of GE. I wouldn’t be surprised to find that you’ve got a pair that’s not properly terminated or broken and dropping you down to 100Mbps.