Torrent download can be slow due to couple of reason. If you are using Azureus you should be able to change the Max. Upload & Download speeds (Change the configuration settings).
A particular torrent can also be slow due to bandwidth restrictions set by the seeder. More seeders you have better speed you get.
Another reason can be your Download/Upload ratio. Certain torernt networks encourage you to share more if you want to get more speed. That means, if you allow others to upload from you more than what download from them you get a >1 (heathy ratio). So you get higher priority when it comes to downloading.
