It may also be possible that your ISP is detecting P2P traffic and is throttling it. While the ethical views on this practice are rather mixed (depending on where in the world you are), it is, unfortunately, something that many providers do actually do to their customers when they see traffic that they don't like.
The problem with this comes in whereas you can't differentiate the difference between legit P2P traffic and illegal P2P traffic, so even things as mundane as Ubuntu software will also get throttled, despite it being available legally for free.
