Torrenting slows down my network speed

I've noticed that when I have Transmission running my network speed becomes very slow. Even if I'm only uploading data (the same is true even if the upload rate is low as 5KB/sec) via Transmission the download speed becomes very slow throughout the computer. It doesn't mattet if I'm browsing using Safari, streaming music in iTunes, if a software is upgrading itself etc they all become slow. As soon as I close Transmission everything speeds up again.

I'm connected through my Airport time capsule.

Cheers.