Thwartedjunk

So I just bought a prebuilt PC from Cyberpower, and so I'm installing games and they are very slow. I'm running an Ethernet cable to it and it's the same one I used for my Xbox One. The speedtest reached 100mbps down, and 85mbps up, but immediately after when I tried downloading a game from Steam it dropped down to a max of 12.5mbps. A few days ago on Xbox I definitely reached at least 100 Mbps when downloading, so I don't understand what is bringing it down to 1/10 of what it should reach.