"Unless you’ve goofed something up, the slowest part of your PHP-based application should be PHP itself. Other parts of your stack may exhibit scaling problems that affect response times, but in terms of raw performance, PHP is the piggy in the middle of your web server and data stores."

I completely disagree. PHP is rarely the issue and I don't know where people get this idea that PHP is slow. Unless you're calculating the flight trajectory to Mars, the main bottleneck of WordPress is MySQL. Fill up you PHP application with microtimers, you'll see PHP only needs milliseconds to do its job even without caching. If there's anything slowing you down, you're waiting for MySQL or Curl or some external application, which is unnecessary most of the time if you designed the application properly.