The synchronous XHR requests are only used in your development environment. For production, use `node browser-require/compile.js`, which will compile all your required javascript modules into a single file. It really doesn't make sense to asynchronously request all of your javascript files on production pages.
Actually it makes a ton of sense to asynchronously require javascript files on your production page. If you load them asynchronously and in parallel, you can often get much better performance than one single request (serially).