"I've most often seen [poor performance] with people running on Windows who 'tweak' their TCP send and receive buffers to be very large in search of higher bandwidth. Buffered data is obsolete data and causes serious problems in the responsiveness of the protocol to changes in the peer state. Since data and requests share the same TCP socket, buffering lots of data means that your request for a block may have to wait 10's of seconds before it even gets transmitted, by which time the peer you are talking to may have decided that you aren't interested in his data after all, or your own client may decide that the peer you are talking to is snubbing you. Either of these is disastrous to download rates." - ( taken from the BitTorrent developer mailing list )
