macOS Sierra slow reboot my MacBook Air (Early 2015) with Sierra (10.12.2) is rebooting very slow (more than 10 minutes). 
How i can fixe this issue? 
How i can see the log file, when MacBook is rebooting? (to understand where is the problem)
After you reboot hold down Command-v to enter verbose mode.

http://osxdaily.com/2007/03/25/always-boot-mac-os-x-in-verbose-mode/