How to slow your connection down, or the Mac equivalent of the Apple ][ speed command.

found here

Configure a pipe that has the appropriate bandwidth limit and delay.
sudo ipfw pipe 1 config bw 16Kbit/s delay 350ms

Attach it to all traffic going to or from port 80.
sudo ipfw add 1 pipe 1 src-port 80
sudo ipfw add 2 pipe 1 dst-port 80

Now traffic coming from or going to port 80 anywhere is limited by the pipe that you specified. Do your testing and once you get frustrated with slow access to the web remove the rules like so:
sudo ipfw delete 1
sudo ipfw delete 2

Finally, delete the now defunct pipe like so:
sudo ipfw pipe 1 delete

put together in one bash script – adjust the kbit/s and delay to suit your needs

sudo ipfw pipe 1 config bw 56Kbit/s delay 500ms
sudo ipfw add 1 pipe 1 src-port 80
sudo ipfw add 2 pipe 1 dst-port 80
#
read -t 1600 -p "Hit ENTER to restore"
sudo ipfw delete 1
sudo ipfw delete 2
sudo ipfw pipe 1 delete