WP Fastest Cache Premium - This plugin creates static html files from your WordPress. When the page is displayed, php and mysql are used. Therefore, the system requires RAM and a processor. If many visitors come to the site, the system uses a lot of RAM and processor, which is why the page is displayed so slowly. In this case, you need a caching system to avoid displaying the page over and over again. The caching system creates a static html file and saves it. Other users reach the static html page.