enabling FileVault2 on APFS appears to be instant (no progress for encrypting drive)

In the past (before APFS) I remember that enabling FileVault would take several hours, during which time it said "Encrypting..." with a percentage progress counter in Settings -> Security & Privacy.

So I was surprised just now when I enabled FileVault2 on APFS on macOS Mojave 10.14.5 on a MacBook Air 2018, and it instantly went to "FileVault is turned on for the disk". I even tried to turn it off again, which was instant too. When I then turned it on again, it was also instantly turned on.

How is it possible that this is so fast?

I ran this command:

fdesetup status

which results in:

FileVault is On.

I also ran this command:

diskutil apfs list

and it displays this for the main volume, so it appears to be

    +-> Volume disk1s1 [...]

    |   ---------------------------------------------------

    |   APFS Volume Disk (Role):   disk1s1 (No specific role)

    |   Name:                      Macintosh HD (Case-insensitive)

    |   Mount Point:               /

    |   Capacity Consumed:         37444063232 B (37.4 GB)

    |   FileVault:                 Yes (Unlocked)