These plugins cache the posts images and pages in your website as a static file and send it to the user when a request is triggered this reduces the load on the server. As you can see it shows the page speed result and the factors slowing down the website in the Yslow tab. For example if the user triggers a request from Florida and your server is in Tokyo think about the time taken to load your page.. The most effective way than browser caching is to make the unchanged content of your websites like images and other CSS files into static files and keep them in multiple hostingnames. Yes the Google's PageSpeed Insights tool also provide the optimized images at the bottom of your page test result..
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