I've helped out a few people recently with several devices, including more than one each of PC laptop, Surface Pro 3, Surface Pro 4, and Surface Book, complaining about Windows 10 taking too long to restart, start up, or boot. On the Surface devices, the white "Surface" word remains on an otherwise black screen, and the circular animation does not show. On a PC laptop, the screen remains black or blue, with no text. Some of these eventually continued starting up, but it may take upwards of 5 minutes. A common culprit of this problem was iCloud for Windows, which inserts itself into the startup sequence before user login, rather than afterwards, when most items start. This is a known problem by Microsoft and by Apple. Prior to Windows 10, the workaround was to enable "Fast restart" in Power Options from the Control Panel. The problem persists even with iCloud 5.1 for Windows, dated 12/7/2015. Workaround with Windows 10 You can still use iCloud for Windows, but disable the startup services. Start iCloud after booting up. To disable, start the Task Manager (Ctrl+Alt+Del) and choose the Startup tab. Note: You can identify items which may start before user login by the "High" Startup impact annotation. OneDrive is one such item, since login protocol (Hello, PIN, etc.) is sometimes accessed before user login, based upon prior connectivity. VPN connectivity items also may be in this list. This is okay. iCloud doing it is not okay, at the time of this writing.