Ali sent me a question that should bother every networking engineer:
Could you explain how Cisco [or another vendor] comes up with the throughput parameters in a products datasheet? For example if a vendor says that "if IPSec is turned on the throughput is 20Mpps", exactly what does it mean? What is the packet size he is referring to and what are the implications here, because very seldom do we have fixed packet sizes in a traffic flow.
The answer, as always, is "it depends". If you're reading a serious performance analysis report, it should document the test procedures, including the packet sizes. If you're getting a "marketing" figure with no further explanation, you can be sure it's been cooked as much as possible. For example, a Gigabit Ethernet link sometimes has 2 Gbps performance (in-and-out) and in case of IPSec packet-per-second values, they are most probably measured with optimal (in this case low) packet size.