Today I learned that flushing the HTML document early can improve performance by allowing the browser to fetch resources as soon as possible.

Also, if you’re interested in learning more about web performance, High Performance Browser Networking is available to read for free on O’Reilly.

The HTML document is parsed incrementally by the browser, which means that the server can and should flush available document markup as frequently as possible. This enables the client to discover and begin fetching critical resources as soon as possible.

Google Search offers one of the best examples of the benefits of this technique: when a search request arrives, the server immediately flushes the static header of the search page prior to even analyzing the query. After all, why should it wait, the header is the same for every search page! Then, while the client is parsing the header markup, the search query is dispatched to the search index, and the remainder of the document, which includes the search results, is delivered to the user once the results are ready. At this point, the dynamic parts of the header, such as the name of the logged-in user, are filled in via JavaScript.

Source: High Performance Browser Networking