I spent ~$35 on new cables and my LAN speed increased 6x
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.
The fact that your old cable was cat5e has no bearing whatsoever on you getting shit speeds before changing cables. The gigabit spec was codified and products were on the market before the cat5e spec was ratified. Gigabit ethernet was literally made for standard cat5. I bet your previous cable was terminated incorrectly, and was only using two of the four pairs, limiting you to 100mbit.