Throttle your loopback interface in Linux to test low bandwith. I was testing an AJAX file upload progress bar and found this helpful.

throttle.sh

# Throttle LO interface
sudo tc qdisc add dev lo root handle 1: htb default 12
sudo tc class add dev lo parent 1:1 classid 1:12 htb rate 20kbps ceil 20kbps 
sudo tc qdisc add dev lo parent 1:12 netem delay 1000ms

# Go back to the way things were.
sudo tc qdisc del dev lo root