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.
They had been collected from various ISP provided modems and routers I've purchased over the years.
At least in here some of the older modems, specially from ADSL-era, only had two pairs in them, so they were only good up to 100Base-T, which is roughly 7MB/s. So maybe check if that's the case and throw those into recycling bin.