When I upgraded to High Sierra, I thought it was time to try a semi-clean install since I had done many years of upgrades. After wiping the drive and restoring from a Time Machine backup, it was time to enable FileVault as I had done for years. It did not go so well this time.
FileVault would not progress and the MacBook was unusable, hanging constantly. This went on for a day until I finally stumbled across an Apple support thread with similar issues and lots of suggestions. None of these helped except one:
sudo fdesetup disable
It did its thing for a bit and everything went back to normal. I’m going to wait a while before trying again, but not until I have a lot of free time.