Why so long to defrag small files ?

[Browser_ice 0]

For as long as I can remember, all the Defraggler versions I have used always had a very slow performance when defraggling small files.  I cannot say what size they are but on my backup HD, when its defraging it, when it hits those small files, it takes 2-3 hours to do a full defrag.  If I disregard those small files (guestimate), the same defrag would have taken less then 30 min.

Why is it so slow for small files?