That's the rub.

As I understand it, the received wisdom is that a constant performance factor doesn't really help you anyway when scaling (you're walking an exponential curve - a constant factor won't help that much). What helps you is having rapid development, so you can remove bottlenecks as you find them (and move onto the next bottleneck).

Basically, what you're after is (horizontal) scalability, not performance. And you can achieve the former more rapidly in a dynamic language (perl, python, pick your poison) than a stricter language (C++, java, C#).

Some notes from a talk by youtube on this: http://kylecordes.com/2007/07/12/youtube-scalability/. In particular: "YouTube is coded mostly in Python. Why? “Development speed critical”

(Of course, there are limits. Having 30 servers instead of 300 is useful. But tweaking for performance (whether that be writing fast, less maintainable perl code or moving stuff to C/C++) generally makes your system less flexible, so is something you want to delay as long as possible).