You need to understand that transferring files via bittorrent is nothing like downloading by http or ftp.
Your transfers are peer-to-peer. That means they’re from somebody just like you and not from a server somewhere. That somebody has a computer like yours and an internet connection like yours. As you download from them, they download from you. You are effectively trading. You need, therefore, to be an attractive trade partner to others, with good upload speed and reliability. You must give in order to get.
Each client connects to a great many others (if it can, if they’re interested in this torrent), and from among those connections it tries to mutually negotiate transfers with the best of those.
If you aren’t among the best that another peer is connected to, it will pass you by in favor of a faster and more reliable partner. You’ll have to try the next-best and see if they will transfer with you. In this way, the best tend to find the best, and the worst have to settle for the worst. It’s a self =regulating system.
If you set your upload speed to zero, you will find that your download speed quickly drops to zero and stays there. You’ll probably read a great deal of whining about “leechers” who take but don’t give. Try this yourself. Set your upload speed to 1 KB/s, which is extremely low. Now watch your download rate plummet. Thereafter ignore the whining, leechers are their own punishment. Your upload rate PER TORRENT needs to be a bare minimum of 8 KB/s, and 30 is a lot better.
If you haven’t learned to present the best appearance that your connection permits, you’ll end up transferring only to slower peers. You need to avoid trying to run too many transfers at once. For most people, that means one seeding task and one download task. MAYBE you can add another download task, but you need to watch your upload rate for all three. If it starts to drop, you are running too many tasks.
That said, everything depends on the other peers in the swarm – how many there are, what sorts of connections they have, how those connections seem to you and how yours appears to them. This is the biggest single factor controlling speed, and it’s completely out of your control.
You have to make sure you’ve done all you can and aren’t doing anything wrong, then hope for good juju regarding the rest. Maximum possible speed varies not only from torrent to torrent, but from moment to moment.