Usually, every website has areas that are requested much more frequently than the other ones. For the sake of performance, it is appropriate to add these areas to HTML cache which will reduce server load substantially. You can do so in "Settings > System Settings > Cache Settings > HTML Cache".

This technology, however, has certain restrictions. Do not add each and every section or web page to HTML cache; rather, you need to analyze the level of demand for each section.

Ensure that each of the website sections added to HTML cache have been analyzed for traffic density.