Most BitTorrent clients use port numbers in the range of 6881 - 6999. Some of the less torrent-friendly ISP uses this fact to slow down torrent traffic. They basically look for communications occurring on these port ranges, and they throttle that traffic. If your ISP takes this approach, it is probably affecting your download and upload speed for torrents.

To guard against this, you should change the ports used by your particular BitTorrent client to be something outside this range. If your ISP is throttling traffic on these ports, you'll get a good speed improvement as a result.