A typical request on an dynamic PHP site can do a lot of things. It’s highly likely that a bunch database queries are performed. On complex pages executing those queries and hydrating them can slow a site down.

The response time can be improved by caching the entire response. The idea is that when a user visits a certain page the app stores the rendered page. When a second request to the page is made, the app shouldn’t bother with rendering the page from scratch but just serve the saved response.