Downloading with a bitTorrent client makes your internet because most downloads expend alot of bandwith.
The BitTorrent protocol relies mostly on users uploading their data. Most internet service providers don't allow their customers much bandwith to upload data.
ISPs decide how many bits(megabits, kilobits, etc) you can transfer per second, and per month. Depending on your provider and plan, your upload and download rate varies.
If you're download a certain file at 8 megabits per second, and your maximum is 16mbps, you are allowed to use 8 megabits more per second. If you use any more, your provider will force your account not to allow any more downloads.
bitTorrent gives you a option to change how fast a certain file can upload and download.
First, go to the main client window.
Second, right click your desired download listed, click "Bandwith Allocation"
Third, you'll see a few options. You can set upload and download speed. If you want to use the internet while downloading, you can set it low for this individual file.
Make sure to set it for both "download" and "upload."
It eats up bandwidth. You can change the bandwidth usage by limiting the upload and download speeds in the settings. Limiting the torrent bandwidth will allow your connection to run smoother.