7.9 is the first version to include a major change in the way that BitTorrent chooses peers in a swarm. Designed by our own Arvid Norberg, Canonical Peer Priority is a way to help peers connect to the swarm faster, as well as reduce the average hop length from you to any other peer in the swarm.
When a bittorrent client joins a swarm, it needs a way to select which peers it connects to. If it chooses poorly, or if there are malicious actors in the swarm, the connections between clients are not well distributed through the swarm, leading to a large number of hops from node to node. That slows down the ability to each client to pass data on to the next.
You can read a more detailed technical discussion of the issues here, along with graphs and figures that drive home how bad the worst case can be. You can read more about graph connectivity here.
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