https://gadgets.ndtv.com/laptops/features/how-to-disable-cortana-on-windows-10-1683223 if you want you can also use regedit to completely disable cortana, but doing so will also break the search bar.
How does cortana slow down your computer? I can't imagine it uses that much memory sitting by itself doing anything.
That's what I was going to suggest. An explanation I have is being particularly active on his system. Don't quote me on it though.
Is it possible you have windows complementary telemetry active? That can cause massive disk usage but disguise itself as other programs taking disk.
I know this is a joke, but windows 10 is way better then windows 8.1. Windows 7 is better then both imo.