That site doesn't know squat about Liferay...

And trust me, just adding "defer" and "async" to your tags isn't going to matter much at all.

You know your site better than some stupid online tool. Are you loading excessive JS libraries (i.e. using angular and react and XYZ on a page)? Are you loading your scripts, etc, correctly (i.e. using the @Component property headers vs embedded <script /> tags in JSPs)?

A tool cannot know what you have done in the site nor why you might have done it. Look at what you have done, what you have delivered. Only you can know that you're loading something you don't need to or that you're not doing things in the most efficient way. But a tool cannot fix a bad implementation or anything, and there's no magic in using additional tags, especially if you don't even know if you can defer or async them in the first place.

Hi David, Thanks for your reply.

(Are you loading excessive JS libraries (i.e. using angular and react and XYZ on a page)? ) --> No, but we have used some plugins for our landing page banner slider.
(Are you loading your scripts, etc, correctly (i.e. using the @Component property headers vs embedded <script /> tags in JSPs)? ) --> Yes, we have used @Component property headers for loading portlet js. 

Well, I knew some of the js files which need to defer or async and I used it and upgrade the performance. 

We also test our site using the page speed insight site of Google. This site suggests us to eliminate render-blocking resources, here resources are only combo files which contain the javascript files for front end library, js_loader_modules, etc... there is also some CSS file from clay CSS from Liferay. The front end libraries and js_loader_modules and other combo files except for the portlet combo document may declared in <@liferay_util["include"] page=top_head_include /> theme portal_normal.ftl file. Can we modify it or remove the files which we don't use?

If we can find the declaration of these files in any way we can optimize the site performance.