These days, nothing worries an internet service provider more than peer-to-peer file trading. Depending on where you live, P2P can account for between 50 and 75% of broadband internet traffic. We mostly have the popularity of BitTorrent to thank for this crazy amount of data going to and fro.
This amount of traffic can raise the ISPs daily costs of delivering service, cause congestion either in your neighborhood or on the ISP's network, and force the ISP to buy increased bandwidth capacity.
But if you've been paying close attention to your BitTorrent transfers lately (or if you've simply been reading the news) you'll notice that ISPs have begun to take drastic measures to slow that flood of data currently clogging up their pipes.
Even though many of them deny it, most ISPs actively engage in traffic shaping, bandwidth throttling, connection denial or some such tactic to keep the amount of bandwidth consumed by high traffic applications on their networks to a minimum. While this does often ensure better performance for everyone in the neighborhood, it can mean painfully slow transfer speeds for those dabbling in P2P -- legit or not.
While there are valid arguments for and against shaping, we're not here to debate. We just want the fastest BitTorrent transfers possible.