We utilize Varnish to cache websites as well, so it will only ever ask the webserver (apache) for content if it is expired within Varnish's cache, or if it is dynamic content (like PHP). This helps us decrease overhead and make the servers more stable, as well as decrease page load time especially for static assets like CSS, JS, Images that do not change often.
Oops! that was a typo. It was a combination of "decrease page load time" and "increase page load speed". ie, decrease the amount of time it takes to load a page, and increase how quickly the page loads.