By downloading one small part of a very large torrent, it will generally download much slower then the same file would in its own torrent.
Selecting files within a torrent is an advanced feature that some bit torrent clients can do, but it will take much longer to download the entire torrent this way, one file at a time. By doing this, most of the torrents data is of no use to you, since your only selecting one of the files, thus making the pieces you need rare.
Here is a good link to help in your understanding of bit torrent protocall…