In order to get he best possible speed, you need to
1.) Be using a tracker with a lot of peers and seeds.
2.) Make sure your connection is not firewalled. A little exclamation appears on the lower right-hand corner if it is firewalled.
In many cases, it may also help to lower your upload rate. On a good torrent, lowering your upload rate will increase your download rate dramatically. Of course, you still want to be uploading at least some. That's what keeps the "community" alive.