In order to reproduce what this will look like in your app/website it can be helpful to artificially slow down your connection to Typekit. This can be accomplished via ipfw (no guarantee that these commands will work exactly as below for other unix variants).
First get the IP address of use.typekit.com by pinging it (for me it is currently 184.108.40.206). Then:
sudo ipfw add pipe 1 ip from 220.127.116.11 to any
sudo ipfw pipe 1 config bw 80kbit/s plr 0.05 delay 50ms
Play with the values 80 to change bandwidth, 0.05 to change packet loss ratio (you can just remove this as well), and 50 to change latency.
When you’re done (note that this will flush all existing ipfw rules):
sudo ipfw flush