#!/bin/sh

/sbin/ipfw add 100 pipe 1 ip from any 80 to any out
/sbin/ipfw add 200 pipe 2 ip from any 80 to any in
/sbin/ipfw pipe 1 config bw 128Kbit/s queue 64Kbytes delay 250ms
/sbin/ipfw pipe 2 config bw 128Kbit/s queue 64Kbytes delay 250ms

http://terminalapp.net/emulating-a-slow-internet-link-for-http/

recover to normal speed with 
/sbin/ipfw delete 100
/sbin/ipfw delete 200
... or reboot

Emulating a Slow Internet Link for HTTP

debug, osx

Bash