For a while now, Chrome has eagerly cleared the screen when transitioning to a new page to give users the reassurance that the page is loading. This "flash of white" is this brief moment during which the browser shows a white paint while loading a page. This can be distracting in-between navigations, especially when the page is reasonably fast in reaching a more interesting state.
Adding rel=preconnect to a <link> informs the browser that your page intends to establish a connection to another domain, and that you'd like the process to start as soon as possible. Resources will load more quickly because the setup process has already been completed by the time the browser requests them.
Say we are a Node.js client, and we’ve made a request to some server. What happens as we’re waiting for that response? How does the event loop know when to put the associated callback on the event queue?