When debugging your mobile application, logging can provide critical information. A few problems arise from the simplicity of iOS’s logging interface. The built-in method, NSLog, is mainly a development tool, but when left in release code it will still be executed. Removing NSLog calls before release can be a tiresome process, but leaving them in will slow down your application.
Read More - http://tech.xtremelabs.com/ios-logging-with-lumberjack/