Eliminate Render Blocking Resources
Render blocking CSS
Browsers download all CSS resources, whether they have blocking attributes or not. That means if you're on desktop, CSS written for mobile devices is still downloaded.
Google says that optimizing CSS delivery -- keeping your CSS file lean, delivering it as quickly as possible, and using media types/queries to unblock rendering -- will go a long way in making this happen.
If you are using a WordPress site, the themes and plugins provided often contain render-blocking elements that are unnecessary upon first page-load. And every time you update the theme or plugin, it will reapply those attributes. These are things you need to edit out again each update.
When a broswer detects a script while building the DOM tree, it has to execute it before it can continue rendering the page. It blocks every other operation until it is fully loaded. This happens especially for external scripts and takes even longer to load.
Many script-heavy sites and applications takes script in order to render each user interaction, thus slowing every other function on the site until it is complete.
Lazy-loading the script until it is needed will also help. And file with more than two dozen unused functions, Google suggests to create a separate file.