Downloads not nearly as fast as speed test indicated?

I recently upgraded my home internet service and at various times throughout the day my downstream speed has tested consistently between 17 and 20 MB/s. However, when I'm actually downloading something, torrents in particular (I use Vuze) the download speed I'm seeing is somewhere in the neighborhood of 150-200 kB/s. Is there a reason for this, and is there anything I can do to correct it? Any suggestions appreciated!