Each level of the tree contains a hash of the hashes from the level below, so to verify the top hash you need only hash all its children (which are hashes themselves). Then you can explore a child, continuing recursively. At the bottom you find a hash of an actual file (or part of a file), then hash the file to check it’s validity.

This allows you to only hash those parts of the tree you actually want to read.

> Big Sur, Monterey or Ventura in default Full Security mode has verified every last bit of their 9 GB SSV.

In this case they're not selective, so the tree approach doesn't save the reads.

If you want to verify the entire image, I don’t think you can get around reading the entire image. Because any part you didn’t read to verify the hash is a part that could contain corruption.

That's around 3s, for their slowest offering.