Here are some links for [libapache2-mod-php] vs [php-fpm] for Apache (both Pro and Con plus install links).
For the first link where the developer switched to [php-fpm] his site could only handle 30 users at a time so his server side framework must have been very slow. As I mentioned the script from the post allows for 2500+ requests-per-second (tested 100 concurrent users) and allows for many more users with tweaking.
The setup for [php-fpm] with Apache is somewhat complex so I would recommend using nginx if you want to use [php-fpm].