Thanks for your post. It certainly does not add up.
What's your current transfer rate speed when doing a speed test while the connection is completely isolated?
There are a lot of factors involved in the download speed, especially when getting data from the internet, I'll give a few examples:
1. Distance or latency of the server where the data is being sourced.
2. Your download is the server's upload, so your download speed will also depend upon the server's upload speed.
3. For file sharing sites, the number of seeders and their as said on #2, their upload speed matters.
4. If your device is on wireless or you have multiple devices on your personal network, these matter to since you are all sharing the same bandwidth https://community.tpg.com.au/t5/Featured-Articles/Easy-Steps-to-Improve-Wi-Fi-Connection-In-Your-Hom...
5. Time of the day in which you do the download, congestion on your ISP and on the routers the data passes through is a factor, you can see that on the latency.
With all the above being said, getting the speed test would definitely help us in investigating the issue and providing resolution.