The build scan asks each directory once

Opening the Builds page spent 805ms walking the search roots, almost all of it on filesystem probes answered by a directory listing it then made anyway. The listing is read first and the expensive checks run only where it says they could succeed: 27-39ms, proved to find the identical set. Built from also says which of its two commits is the checkout's current head.

Opening the Builds page took noticeably long before anything appeared, and 805ms of that was the directory walk — against 116ms for a plain walk of the same roots.

Neither cause was algorithmic. The walk asked before it looked: every directory ran the checkout test and then the application test, and the latter probes each executable name in turn, so a directory that is neither paid for the whole list before the readdir that would have answered in one call. The listing is read first now, and the expensive checks run only where it says they could succeed. The walk is also breadth-first and bounded-concurrent rather than strictly serial.

walkRoots  36ms   readVersions 273ms   materialize 19ms
walkRoots  27ms   readVersions 302ms   materialize 18ms
walkRoots  33ms   readVersions 303ms   materialize  9ms

• Proved equivalent rather than assumed. A longhand serial walk of the same roots, with the same predicates written out, finds the identical set — one checkout, two application directories, nothing on either side only — and takes 3358ms doing it.
• Results are gathered per input index rather than pushed as they land. The scan dedupes by root, so a non-deterministic winner would be a non-deterministic page.
• The remaining cost is reading each executable's version resource, which is now most of the scan.
• The Built from column also stops reading as a range: abc1234 → def5678 looked like a span of commits when it meant "compiled from the first, and the checkout has since moved to the second". The second is labelled, and a tooltip says it in words.

• The numbers are this machine's, with four search roots and 1,433 directories under them. A tree of a different shape will differ.
• Nothing here speeds up the version reads, which now dominate.