Chapter 9 High Availability for FortiOS 5.0 : HA and failover protection : Failover performance : Link failover performance
  
Link failover performance
Link failover time is controlled by how long it takes for a cluster to synchronize the cluster link database. When a link failure occurs, the cluster unit that experienced the link failure uses HA heartbeat packets to broadcast the updated link database to all cluster units. When all cluster units have received the updated database the failover is complete.
It may take another few seconds for the cluster to negotiate and re-distribute communication sessions.