HTTP/2 Server Push allows a website to push content to a browser, without having to wait for the HTML of one page to render first. In conjunction with the concurrency support built into HTTP/2, Server Push is able to reduce the time needed to load your website.
HTTP/2 is the new generation of the venerable HTTP protocol that powers the web. Among its most powerful features is server push, a way for web servers to send resources to the browser before it even realizes it needs them. This avoids the usual HTTP request/response cycle which happened for every script or stylesheet on a page. You can enable this feature in Settings→Optimization→General: Enable Server Push
Requires a web server that supports HTTP/2.