There can be a few reasons you would get a slow torrent on an otherwise fast internet connection. First would be if there aren't very many seeders on your torrents, which would make downloading slow for every peer. Second would be if you haven't done any port forwarding or done it incorrectly and your torrent client and router don't have the ability to negotiate it themselves. Third would be if your ISP is throttling torrent traffic, which does happen, in which case you can't do very much about it.