...at Limewire.com, so you could consider it the definitive answer:

"Q: Why do some things take so long to download?

A: The speed at which a download takes place is limited by both the bandwidth of the downloader and by the bandwidth of the uploader. A file cannot by transferred faster than the uploader is capable of sending it, nor can it be transferred faster than the downloader is capable of receiving it. If you have a T1 Internet connection and you are trying to download a file from a person with a 56K modem, your transfer is going to take place rather slowly. Furthermore, some clients limit the speed at which they will upload files for the sake of saving bandwidth."

Best regards.