Bulk directory reads across eight workers. Eight is the measured sweet spot.
Memory
About 60 bytes a node, depth-first, in flat arrays
Rescans
Replays the filesystem event log. Six named conditions force a full walk.
Accounting for the gap
Every disk tool shows a difference between what it adds up and what the system reports as used. This one names it: purgeable space, local snapshots, folders it cannot read, and other volumes in the same container.
The user folder is a firmlink onto another volume, so walking it naively counts the same bytes twice.
A full scan of this Mac: 838 GB in 48.4 seconds, with the unreadable folders counted and named rather than ignored.
