[SoLiDUS]

Bittorrent works very simply: you must share bits of the file with another in
order to get anything. When you start out, you have 0 parts of the file. Now,
suppose the file is 20 parts long: a few people will have to "give generously"
initially so you may begin the SHARING process, which is simply trading one
piece you have that the other person doesn't for something he has that you
need.

Near the end, you will only be able to acquire the pieces from a small number
of people, hence the speed decrease. Think of the distribution as a bell curve:
most people will be within a deviation of the average and only a very small #
will find themselves at either end. This is why the speed is slow during the first
and last moments of transfer.

Unless there are several high-speed seeds offering the bits...