Well there could be many factors causing a slow download speed. One factor could be the presence of a router. If you have a router, you should do something called "Port Forwarding" which opens the ports that Bittorrent uses (6881-6999) in your router, allowing a faster download speed. If you have a router and haven't done that yet, post the type of router (Linksys, D-Link, etc.) and the model number of the router and I can give you a step by step guide to port forwarding. Another factor could be the amount of seeders that there are for that torrent. The more seeders there are, the faster the download speed will be for that file. Hope I could help.

P.S: This should really be in BT Support.