Before you try anything else, get that Realtek (rl0) NIC out. It's a fact that these don't perform well - at least not with FreeBSD/m0n0wall (see e.g.
http://m0n0.ch/wall/list/showmsg.php?id=247/94). Your other (2 port gigabit) card seems to have Intel chips, and those are very good. Put LAN/WAN on em0/em1 and then try again.