[nik@nik-fedora28 ~]$ time sudo guestfish --ro --format=raw -a /dev/sdd run : list-filesystems
/dev/sda1: vfat

real	0m0.621s
user	0m0.347s
sys	0m0.189s

[nik@nik-fedora28 ~]$ time sudo nbdkit file file=/dev/sdd --run 'guestfish --ro --format=raw -a $nbd run : list-filesystems'
/dev/sda1: vfat

real	0m5.409s
user	0m0.566s
sys	0m0.424s