With the ASA version 8.4 Cisco introduced port-channels (ASA5510 or higher) what is a really nice feature because you can share the load over more interfaces. When it comes to a physical interface the incoming or outgoing traffic is processed through Fifo queues and RX/TX-rings per interface, when theses queues or rings are full nothing can pass through. So its not always related to CPU load of the device also the phsical interfaces are in the scope when it comes to performance. With port-channel you share the load over more Fifo queues and RX/TX-rings but keep in mind about the port-channel balancing mode.
Physical interface configuration
interface GigabitEthernet0/0 description GigTrunk-Po1 channel-group 1 mode active no shutdown exit interface GigabitEthernet0/1 description GigTrunk-Po1 channel-group 1 mode active no shutdown exit interface GigabitEthernet0/2 description GigTrunk-Po1 channel-group 1 mode active no shutdown exit interface GigabitEthernet0/3 description GigTrunk-Po1 channel-group 1 mode active no shutdown exit
Port-channel configuration with VLAN sub-interfaces
interface Port-channel1 no shutdown exit interface Port-channel1.100 description Server VLAN vlan 100 nameif SERVER security-level 100 ip address 10.1.0.254 255.255.255.0 standby 10.1.0.253 mac-address 0a00.0a00.0011 standby 0a00.0a00.0021 no shutdown exit interface Port-channel1.200 description Clients VLAN vlan 200 nameif CLIENTS security-level 100 ip address 10.2.0.254 255.255.255.0 standby 10.2.0.253 mac-address 0a00.0a00.0011 standby 0a00.0a00.0021 no shutdown exit
For more information read the Cisco ASA 8.4 Configuration Guide – EtherChannels