Why is fsck_hfs slow on *journaled* HFS+

Sometimes my external drives get uncleanly detached, and at the next connection a disk check is necessary. What surprises me now is to see a slow disk check process *on a journaled filesystem*. According to the manual, that requires the -f option, but that seems incorrect. Isn't the point of journaling that you *never* need a full disk check? In detail, macOS started fsck automatically with -y:

$ ps auxw|grep fsck
root              3792   1.1  2.3  6429668 389400   ??  U     2:09pm   0:05.10 /System/Library/Filesystems/hfs.fs/Contents/Resources/./fsck_hfs -y /dev/disk3s2

Then I interrupted the process and re-run it by hand without -y for greater control, as I usually do:

sudo fsck_hfs /dev/disk3s2
** /dev/rdisk3s2
   Executing fsck_hfs (version hfs-407.50.6).
** Checking Journaled HFS Plus volume.
   The volume name is MyPass4T-TM2
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.

FWIW, this is on a journaled HFS+ sparsebundle, that I use for Time Machine, hosted inside an external filesystem (ExFAT).