It's crucial to test your apps against a slow cellular network. This affects how well your app performs and if it even works. For example, it is a common problem for apps to block when accessing the network. This problem is hidden on a fast connection.
Network Link Conditioner slows down your machine's network connection so you can test your app in the iOS simulator thoroughly. To install Network Link Conditioner with the latest version of Xcode (v4.5 as of writing), go to Xcode Menu, choose Open Developer Tool > More Developer Tools. This should bring you to Apple's developer portal where you can download a set of tools including Network Link Conditioner.
Once it's installed, it appears as a panel in System Preferences. Select the profile “Edge, Lossy Network” and switch it on. Voilà!