Anyone use iperf?

I'm trying to test throughput on a mono system.

I connect PC-a to PC-b with a crossed cable and I get 94 Mbps

PC-a ==== LAN ==== mono ====  DMZ ===== PC-b gets me 93.9 Mbps

but...

PC-a ==== LAN ==== mono ====  WAN ===== PC-b gets me 19.5 Mbps

Is there any reason why the result is so low? What do I need to do to
increase this? The command used is iperf -c PC-b-ip -w 250k

sai