Unexpected Progress Bar after login
That is the boot process happening (or perhaps stuck and not happening ) between the file vault unlock screen and the full macOS being booted. If you powered off the machine during the last reboot, this full disk check could take a long time if you don’t have problems. If you do have problems, it might freeze or never finish. 

This isn’t much different than the Sierra process, so the way to diagnose is the same. 


https://support.apple.com/en-us/HT201573


If you start up in verbose mode, you can see if it stops in a specific spot. 


https://support.apple.com/kb/PH25421