When you develop websites or apps your server is usually inside a local network or even in your own computer. To get a closer look to the real user experience you need to slow your network speed down. This is called network throttling and in Mac OS is really easy to do.
Head over to the downloads page in Apple's developer site and search for
Hardware IO Tools
If you have Xcode, you can get to the downloads page through Xcode > Open developer tool > More developer tools
Download the latest image, open it and you will find a file called
Network Link Conditioner.prefPane. Double-click it and will be added to your system preferences panel.
Now you can open the Network Link Conditioner in your system preferences and select the connection you want to simulate.
Do it, test how your site really behaves using a 3G connection or normal broadband, it might not render as fast as you thought. In addition, this is a great way to make sure you avoid race conditions.
Enable your device for development and a Developer item will pop up in Settings, allowing you to enable the network link conditioner, among other features.