Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : About the FGCP : Load Balancing
  
Load Balancing
Active-active HA load balances resource-intensive virus scanning, web filtering, intrusion protection, Application Control, email filtering and Data Leak Prevention operations among all cluster units to provide better performance than a standalone FortiGate unit. If network traffic consists of mainly TCP sessions, the FGCP can also load balance all TCP sessions to improve TCP performance in some network configurations. You can also load balance UDP sessions. You can use accelerated FortiGate interfaces to also accelerate HA load balancing and HA load balancing schedules can be adjusted to optimize performance for the traffic mix on your network. Weighted load balancing can be used to control the relative amount of sessions processed by each cluster unit.