The differences come down to the processor speed and busy time.
A stock Mandy install includes a huge amount of superfluous software, all taking cpu time.
A bare box firewall/router can be booted from a floppy disk, eg LEAF
or even loaded into a new LinuxBIOS rom, eg www.linuxbios.org
This becomes a streamlined device, only what is needed is installed.
The performance then depends on the amount of RAM, for holding connection tracking data, fragmentation queues, VPN decodes etc.
This article gives a greater description http://www.linuxjournal.com/article.php?sid=5826