Async functions and the await keyword are a new addition to JavaScript. In my opinion, it makes your code cleaner and easy to read by avoiding the need to create a promise chain.
How fast the browser builds your app depends on how you build it. The first thing the browser receives is an HTML document and other assets like image, CSS, javascript.
A headless CMS is a back-end only content management system (CMS) built as an editing interface and database that makes content accessible via an API for display on any device.