amule has such behavior when it starts finishing all the downloads: i.e. if you add ten very common files, then you will get great download speeds, since every file will have plenty of sources, and any chunk is useful. But when you start downloading and getting chunk after chunk, then the non-useful sources are more for every file, and the the files start completing, and it's harder to get just the chunks you need, so the download speed goes down. At least, that's what I've observed in my (a|e)mule.
Regards.