Currently the file system watcher will skip watching ignored directories in trivial cases:

/foo

However, as soon as there is something "tricky" like an exclude pattern, it doesn't:

/foo
!bar

(/foo is no longer skipped, because in the mind of the ignore matcher, we might find unignored stuff underneath it...)

However there are a bunch of simple cases like above where we can safely say that anything under /foo must be ignored, because any other patterns come later in the file and will never be processed.