I have a similar problem. It seems the latest Microsoft updates have caused a drastic slowing of internet speed. In my case, I have 1 Gig service, and only getting 300 Mbps now. Happening suddenly on two Windows-10 computers. I have iPhone, iPad and other apple devices that do not have the same problem, and are getting the full download. My Apple devices ARE NOW GETTING TWICE THE INTERNET SPEED EVEN ON WiFi than my Windows 10 computers are getting on Ethernet.
I had Comcast check my external and internal lines for speed, and had Dell and Microsoft work on the problem, but with no change.
FURTHERMORE I'M FINISHED WITH MICROSOFT!
I don't have time to wait. I am frustrated, and in the process of changing over to Mac solutions from Windows. Too many missteps by Microsoft. It will take me several years to switch, but I made my first step this week. All my Apple devices are getting the full internet speed.