i could be downloading some type of torrent like ubuntu or something and it slows down my internet even if its only downloading at 200 kb/s or less. and uploading at like approx. 3 kb/s. and no my internet is not slow it averages at like 900~1000 kb/s. i did a speed test to prove it.
Torrents use a lot of bandwidth no matter how fast or slow the download is because they have multiple people connected at the same time. Whether you are downloading directly from them or just have an idle connection to them, they are using bandwidth.
EDIT: Torrents are not near as bad as most P2P sites, such as Kazaa. Of course, it always depends on where you get the files from (the original source, not the application).