speedup macOS Sierra startup
I don't know if this is reliable, but I saw on macxdvd.com (I am not recommending their software) that if boot time is slow on Sierra, to try the following:


  Many users reported that updated macOS Sierra boot-up time increased a lot. How to speed up macOS Sierra? It's quite easy. Head to System Preferences > Startup Disk > primary disk and click "Restart" the Mac computer. If it doesn't fix slow Mac macOS Sierra after upgrading to macOS Sierra, reset PRAM.


Apple has instructions to reset PRAM:


  Shut down your Mac.
  
  Locate the following keys on the keyboard: Command (⌘), Option, P, and R. 
  Turn on your Mac.
  
  Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
  
  Hold these keys until the computer restarts and you hear the startup sound for a second time.
  
  Release the keys.
  
  After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.


Please comment if this had any effect.