When a page is rendered, php and mysql are used. Therefore, system needs RAM and CPU.
If many visitors come to a site, system uses lots of RAM and CPU so page is rendered so slowly.
In this case, you need a cache system not to render page again and again. Cache system generates a static html file and saves. Other users reach to static html page.
Discover the features of WP Fastest Cache Premium Nulled and be aware the difference with other plugins.
Demo : https://www.wpfastestcache.com/