What is “Time to First Byte” & Why is it Important

Before browsers render any content from your websites, they need to parse the HTML markup into a DOM tree. The HTML parser is paused by render-blocking resources — any external stylesheets or synchronous JavaScript tags that it encounters.

Scripts (eg: <script src="main.js"> ) and stylesheets (eg: <link rel="stylesheet"> ) are both render blocking resources which delay your page from showing your content to visitors (See: First Contentful Paint and Largest Contentful Paint). It is recommended that any non-critical JavaScript or CSS is deferred to speed up the loading of your website’s main content.

Pagespeed Insights suggestions to eliminate render-blocking resources on CVS.com