When I have used all the investigative methods mentioned by others in this thread... it has almost always led back to Windows 10 itself. Typically it has something to do with some sort of update process gone bad. This is the primary reason why I hate the control that the All Knowing A$$hats at Microsoft have taken away from users. I am shocked that no one has mentioned this earlier in the thread.
yes windows10 forcing those background updates kills my speed every single time