There is a reason for me (testing) to turn off FileVault encryption on the boot drive on my Mac running macOS High Sierra (long story, off topic).
So, I turned it off in System Preferences. I also ensured that the system doesn't go to sleep.
It started out with saying it would take 3 hours. Then when I looked again a while later it said it would take 'more than one day'. After reading horror stories on this taking forever, I started looking into why and into ways to speed this up.
And what surprised me was that the system is 98% idle. Core i7 with 4 cores doing nothing. So, somehow the system decides that it should not do anything about it now. But under what circumstances does it do anything about it? If nobody is logged in? If there is no activity of a certain kind? Or the other way around: if disk data is accessed do it anyway and do the rest in the background, slowly? I also looked for a process to renice, but there is no obvious candidate.
Is there a way I can get the system to actually use CPU power to do the decrypting?