In order to achieve this you need to add defer attribute your script and CSS reference. Above-the-fold-content issue actually makes your actual page content to wait for loading until scripts and style sheets referenced inside <head> are loaded.
For example if you have a script referenced like
<script type="text/javascript" src="/scripts/customscript.js"></script>
,
you should rather write it
<script defer="defer" type="text/javascript" src="/scripts/customscript.js"></script>
You can use the same approach for your CSS files as well, but it is not recommended to use it to your main stylesheet. If you have some CSS files which does not require to be loaded when your page gets rendered, you are OK to use it, like
<link rel="stylesheet" type="text/css" href="/css/notonpageload.css" defer="defer" />
Besides this, you can improve your page speed by taking a few more steps. for example:
1. Adding a version to your static contents
<link rel=”stylesheet” type=”text/css” href=”/css/notonpageload.css?v=0.0.2″ defer=”defer” />
<script defer=”defer” type=”text/javascript” src=”/scripts/customscript.js?v=0.1.0″></script>
2. Moving script files which are not required to be loaded when your page is loading to bottom of the page before end of <body> tag. For example a script file which is called when user clicks a button on your page.
And finally, the most important thing…
WordPress is designed for optimized performance. If your site is “really slow”, I would recommend you to check the following areas:
1. Is your WordPress version up-to-date?
2. Are all your plugins up-to-date?
3. Are you using a lot of plugins? More plugins you have, slower the site goes
4. If you plugins you are not using, remove them completely.
5. Keep your database clean. If possible remove posts which are in trash and you don’t need them. Heavier the database, slower is your site performance.
Hope above suggestions will help you improve the performance of your site. Let me know if you further assistance.
Thank you!