Restart screen features a progress bar: what is it?
This is the Mac restoring from safe sleep (aka hibernation). The contents of RAM was previously copied to disk and is now being restored. For HDDs, this process can take a while.


  When your Mac wakes from sleep, you might see a progress bar on its display. This is normal behavior that means your Mac is waking from Safe Sleep.


If you see a progress bar when your Mac wakes from sleep - Apple Support