Just out of curiosity, what Internet service provider are you using? Some ISP’s are well known for trottling P2P traffic and this could easily be the case here if have not had any downloads with a sustained transfer rate of over 100KB/s for your connection.
Going by the number of seeds vs. peers you had for the connection above, 20KB/s to 40KB/sc seems about right to me. Basically, if a torrent consists of a lot of peers, but only a handful of seeds, then with the average uplink of around 256k, the average transfer speed you can expect would be around 25KB/sec to 32KB/sec. The reason the torrent starts off so quick is due to what peers it can connect to for data as the download progresses. For example, when you first start a download, virtually every peer that has any part fo the file downloaded can act as a potential source to download form. However, as your download progresses, the number of peers that have parts of the file that you don’t decreases. However, if the torrent has a lot of seeds but only a few peers such as 100 seeds and 20 peers, you can easily expect speeds of in exceess of 100KB/sec.
A fairly simple way to check if your ISP is throttling your P2P traffic is to download a large file that consists mainly of seeds and see if the download speed is sustained through the download. For example, the OpenOffice package torrent (>90MB) would make a good test as the torrent mainly consists of seeds. If you don’t see a sustained speed of over 100KB/sec (in my case around 200KB/sec for my 2Mb connection) for this torrent, then either your ISP is trottling P2P traffic or your connection has a high contention on it.