Most of us are downloading torrents ( for legitimate purpose ) and it’s becoming such a problem because it’s using a lot of bandwidth. The Internet service provider are well aware of that problem and can not afford upgrading the infrastructure to a much more capable solution. Their way of fixing the problem is to throttle your internet speed when you are downloading a torrent. Let’s not be blind folks 98% of the traffic on torrents are illegal content such as music, movies, games and programs. I have seen legitimate torrents such as version of Linux and such but most of it are illegal; plain and simple.
The Internet service provider are often using the port number and the protocol used to detect the kind of traffic on their network. They are often using ( DPI ) deep packet inspection to determine if a packet contains peer to peer control messages, If it does the ISP will throttle the speed of the download/upload. ( you can view Glastnost white paper on traffic differentiation here ).
Glasnost provided a link where you can test and verify if your ISP is throttling your traffic. The test takes about 8 minutes to complete.