
The downloaded files can be really big.So, having a more upload speed in your ADSL/cable connection will also speed your downloads.
In general, your download speed corresponds a lot to your upload speed.
The torrent's downloaders' total download speed matches the total upload speed. As there is no central server, your download speed is directly dependent on uploads from others.
The low speed or inactivity may also be due to low amount of seeds/peers in the torrent, or their slow upload capability. So, it is quite possible that torrent remains active with "downloading" status, but is not actively downloading anything as there is nobody to download from.
It is quite possible with a "new" torrent or one with "small" swarm (only a few peers), that the missing pieces are not available. There are a few reasons why you might be experiencing slow downloads.