had the same issue and actually managed to solve it without having to disable the Fastboot feature.

This is what I did:

1) Enabling every device I had disabled before: on Device Manager I disabled the Wifi and Bluetooth adapters because I wasn't using them. Enabling them again managed to reduce the issue by ~50%

2) Wiping the C:\Windows\Prefetch (or "%windir%\Prefetch" if you can't find it) folder. These are the files used to speed up the OS boot, but probably some were corrupted and generated the issue preventing the Fast boot to work. I deleted every file and folder inside Prefetch leaving only "Layout.ini" left. With this I totally managed to resolve the issue, I'm around at the 10th correct Fast Boot!