Deferring loading of JavaScript functions that are not called at startup reduces the initial download size, allowing other resources to be downloaded in parallel, and speeding up execution and rendering time.
Like stylesheets, scripts must be downloaded, parsed, and executed before the browser can begin to render a web page. Again, even if a script is contained in an external file that is cached, processing of all elements below the script is blocked until the browser loads the code from disk and executes it. However, for some browsers, the situation is worse than for stylesheets: while JavaScript is being processed, the browser blocks all other resources from being downloaded. For AJAX-type applications that use many bytes of JavaScript code, this can add considerable latency.