You're comparing a custom forum written in a pet low-level language to PhpBB and concluding that the language is the reason for the performance difference? Clearly you have a lot to learn about web application performance.
One of the reasons, not the sole reason. The point I was trying to make is that "just ignore performance and use an expressive language" is not always a sound strategy - even for web applications. And yes, my example is probably a poor one.