When developing websites and such its good to know how loading will perform for users with slow/laggy connections. On FreeBSD or OSX you can model this using ipfw.
sudo ipfw pipe 1 config bw 50KBytes/s delay 300ms sudo ipfw add 1 pipe 1 src-port 4000 && sudo ipfw add 2 pipe 1 dst-port 4000
This will add a 300ms latency and a 50kBytes/s bandwidth restriction to our app running on localhost:4000. Finaly we can delete the restrictions using the following command to delete the pipes we setup.
sudo ipfw delete 1 && sudo ipfw delete 2