Ever since Google started to consider site speed for search rankings, webmasters and bloggers started to improve site loading speed.
You can check the best tools to improve site loading speed, FYI, you can also use Google Analytics to check site speed.
Why To Add PHP Flush Function?
PHP function flush() allows you to send your partially ready HTML response to the browser so that it will start to fetch the components while your backend is still busy with the rest of the HTML page. You can experience this effect when you have busy backends or light frontends.
Though the speed increase will actually depend on the server-side processing, page weight, and size of your CSS files. It is a simple optimization, and nothing wrong in doing it.
Where To Add PHP Flush Function?
<?php flush(); ?>
This is a simple tweak, and do tell us if this works for you.