Slow initial Chkdsk /R on new 18TB WD Elements

Why is the //Windows 7 64//Chkdsk /R// sector-by-sector-verify performance
on WD Elements 18TB external USB drive so slow ?
Who else has tried Chkdsk /R on this WD 18TB external USB hdd hard drive ?

Chkdsk /R on new and empty WD Elements 18TB is now at 72% ,
after 235 hours (almost 10 days) .
72 percent complete. (1517398666 of 2197240073 free clusters processed)