The Async tag is used to improve page load time. It creates an empty HTML element, and after the page has loaded, a little script makes an asynchronous request to the server to render the template. This approach is also known as "lazy loading".
The Cache tag is used to improve page load time. It caches the rendered template using WPObjectCache, which stores it in the database by giving it a unique name and expiration time. This supports persistent cache plugins, such as Memcached, Redis, W3 Total Cache, and LiteSpeed.
The request
The Timer tag measures the time it takes to render a template section.