@haggan
If you boot it in verbose mode, do you see a repeating message of, "Too many corpses being created."?
If so, it's a bad cache file; boot into recovery mode and do the following (courtesy of a GSX notification I got at my Mac repair job; this can be a thing that started with 10.13.4, but we've seen it in the shop all the way up to the latest High Sierra update):
Restart your Mac and hold down Command-R to start up from macOS Recovery.
If the startup drive has FileVault turned on, open Disk Utility and proceed with the next step. If FileVault is off, skip to step 5.
Select the startup drive and click Mount in the Disk Utility toolbar. When prompted, select a login name and enter the password. Then click Unlock to mount the startup drive.
Quit Disk Utility.
Choose Utilities > Terminal from the menu bar.
Type this command in Terminal: cd /Volumes/Macintosh HD/var/db/caches/opendirectory/
Modify the command to reflect the name of the startup volume if it's not Macintosh HD. Remember to use an escape character before each white space in the command path.
Press Return.
Type this command in Terminal: mv ./mbr_cache ./mbr_cache-old
The mv Terminal command is safer than the rm command. Errant white space in an rm command can destroy the user's data.
Press Return.
Quit Terminal.
Choose Apple menu > Restart.
The first startup after running these commands takes longer than usual as the cache is recreated. Subsequent startups will take the normal amount of time.
