[librechroot] sync can be slow, even on btrfs
If a chroot contains a whole bunch of data (like, say, the 4GB mozilla-unified hg repo), then
find "$1" -mindepth 1 -xdev -depth -inum 256 -print0 might take a while to scan it for nested subvolumes, even though there's not much to find.
See also: my proposed
btrfs_subvolume_delete that I submitted to arch-projects February 2017.