The peers with less of the file and further from the seeders will tend to have higher bandwidth as they have more peers they can download from. The vast majority of the network is after only one piece of the file and few peers have it. This collapses the network into a more tree like structure (rather than the desired mesh) which limits the network bandwidth. Whether this is enough to degrade the swarm to the point were it is not useful for streaming or even causes the file to become incomplete is another question, but it does impact bandwidth. I can give an example if this still isn't apparent, but it'd take me a bit to write up.