There can be a few reasons you would get a slow torrent on an otherwise fast internet connection. First would be if there aren't very many seeders on your torrents, which would make downloading slow for every peer. Second would be if you haven't done any port forwarding or done it incorrectly and your torrent client and router don't have the ability to negotiate it themselves. Third would be if your ISP is throttling torrent traffic, which does happen, in which case you can't do very much about it.
Y A is absolutely correct. We as a community might have some say over the speed of a given torrent, but if you are getting copies from Bulgaria or Greece (both of which I've seen on lists of peers) the connections may not be very strong (a lot of packets dropped). And some ISPs try to discourage it by throttling it.
You can improve it by making sure your bittorrent client is properly configured, but since bittorrent is a protocol, all of the clients will give you ROUGHLY the same speed and so forth. There will be SOME improvements but not major ones.