Emulate slower Internet speed. (Apple also provides a free Network Link Conditioner pref pane that does the same thing.)

makefast.sh

#!/bin/bash

# Make Internet connection fast again
sudo ipfw delete 1
sudo ipfw delete 2
sudo ipfw delete 3
sudo ipfw delete 4

makeslow.sh

#!/bin/bash

# Note that for this to work with localhost, use the machine’s current IP, such as 192.168.0.1
sudo ipfw pipe 1 config bw 640KByte/s delay 100ms
sudo ipfw add 1 pipe 1 src-port 80
sudo ipfw add 2 pipe 1 dst-port 80
sudo ipfw add 3 pipe 1 src-port 443
sudo ipfw add 4 pipe 1 dst-port 443