I adore OpenBSD because it's so lightweight, but I dropped it for FreeBSD when the same hardware couldn't max out a 1 gigabit connection running OpenBSD but could with Free. Since then they have made network changes so I'll probably try again even though the bar's moved and now I need 2.5 gigabit connections to be saturated. Happy to give it a chance anyway.
I just remember considering it for routing/firewalling duties about 2007, grabbing then-current performance optimization guide, and finding whole chapter in how to force irq sharing so that interrupts from any NIC will trigger driver code from all NICs...