Hello! I've just created this account because I just have the same problem, and I have a suggestion about what is causing it.
So, since last update of Avast on my Windows 8.1 computer I noticed the "Avast Service" running at around 30% of my CPU all the time. I tried the reboot fix, it worked.
Then the issue came back.
EVERY time my 8.1 computer boots with the fastboot method the problem is back, EVERY time I do a manual reboot and it's back to normal.
tl;dr looks like a fastboot problem with Avast Service with Windows 8.1
I hope it will help solve it quickly.