I've been battling this issue on and off (sometimes it just wasn't worth it) for months now. Ever since I upgraded from windows 8 to 8.1. This is my work computer and it's mostly just a management workstation so I didn't spend a whole lot of time looking into it at first. Today I finally decided something had to be done. It was taking WAYyyyyyyyyyy too long to get things done. Even responding to an email or opening a web page was getting cumbersome.
I looked at all the usual suspects:
-Did a malware scan
-Changed windows search so it was looking at basically nothing
and a few other registry edits found on random boards.
What really did it for me was watching the resource monitor and noticing that it was reading heaps from \Windows\SoftwareDistribution\Download. So - I decided to delete the folder. Low and behold - disk utilization dropped to nil.
I'm happily now running at 1% disk usage.
Hope this helps someone out there.