Originally Posted by Vuze Wiki
The average swarm speed is calculated by measuring the interval between have-messages and the amount of data they represent you get from your peers (the estimated download speed column in the peers view is based on that information, too). Then the calculated peer speed (including your own) is divided by the number of peers you're connected to.
Please note that this is just a rough estimate and not very precise for torrents with large piece sizes or when you're only connected to few peers.