When a visitor to your site visits a page, it can takes around 200 – 500 milliseconds for the backend server to put together the HTML version of the page. During this time the browser remains idle. The flush() function in php will help by loading the partially ready HTML page to the browser and it can start fetching the components, while the backend server is busy with the rest of the contents.
To insert the flush() function in your site, open up the header.php file and find the </head> tag and insert the <?php flush(); ?> function right after it.