Although this may be a hardware problem, such as your motherboard heading south, let's try removing all the other options first.

You mentioned running speed tests with both a phone and the problem desktop. Please borrow a friend or family member's laptop and test using both a wireless connection and a ethernet connection. That will give you a sure answer as to whether it's the desktop's problem. In addition, the secondary computer will give you the ability to download programs or drivers to fix your computer.

Next, do you get the same results if you run the speed tests with a different browser, such as Firefox?

You also didn't tell us whether you're downloading Torrents or sharing files. That gives us a clue as to what may be affecting your machine.

Next, because it may be malware causing the issue, click on the link and follow the instructions in the order they are given. You'll probably need the borrowed laptop to download the scanning tools.

“Expand” the post titled: "Please try this" at the link below:
http://www.cnet.com/forums/post/f742c795-5881-433b-a29b-6d758efe5cd3/

And finally, just for grins, try running Chkdsk in "repair" mode to find any damaged files/free space and correct them. Start by opening a command prompt as an admin. Once there, type chkdsk /f /r /x, leaving the appropriate spaces between the letters as shown, then press the Enter key. You should see a note that it can't be run till a restart is done, type "Y" for yes, then press the Enter key again. Close the command prompt and restart the computer. Chkdsk will run upon restart and will take quite a while to finish.. Don't panic when the progress bar stays at 10% for a long time. It's fairly normal and the Chkdsk process may take a couple of hours.

When it's finished, it will boot to your normal startup/lock screen and you can login normally and then see if things are working correctly again.

Hope this helps and let know more.

Grif