It probably does not apply to your case, but a typical problem with torrent (and other p2p protocols) are routers, that cannot handle many simultaneous connections or that have issues if many connections are initiated in a short period of time. People often realize that they are affected by this issue, when they switch from Windows to some other operating system. The reason is that Windows has limits for simultaneous connections and new connections per second, and at least in earlier versions of Windows those were set to ridiculously low values per default, so people never realized that their router doesn't handle many simultaneous connections well.
A symptom of this is, that not only the p2p downloads are slow, but also other internet connections like web-browsing are affected and show very low speeds while p2p software is running.
In such a case, it actually increases the p2p-download speed, if one limits the number of connections in the p2p software. Some p2p programs even offer a limit on the number of new connections per second (actually I only know one: mldonkey). I'd suggest to start with low values for these settings and then to increase them until issues arise (and then go back a little bit - in order to have stable conditions).