To investigate: _dirstat_np

[mikekazakov]

While Apple never released an official API for fast calculation of directory sizes, there's a internal function that can be used to do it: _dirstat_np.
See:
https://opensource.apple.com/source/Libc/Libc-1244.1.7/libdarwin/dirstat.h
https://opensource.apple.com/source/Libc/Libc-1244.1.7/libdarwin/dirstat.c.auto.html
Worth investigating this function and whether it can be used in Nimble Commander.