I'm not talking about the speed at which code executes, I'm talking about how many users you can support with a single server. Or how many servers do you need to support N users.
axod, you set yourself up with the 10 to 1 comment. I work every day in C# and we are currently moving all development to Java, at home I code in ruby. I will tell you from experience that your language plays a very small part in how fast your web app works. I will say without a doubt that programmer competence is the real factor. How the code is written will determine more how it runs. But for the sake of argument I can guarantee that if you wrote a web app the exact same way in Java and then ruby, or basically any modern language, one would not consume 10 times the resources of the other. Other factors will start to bottleneck you long before that, bandwidth, memory, cpu, etc.
I'm not talking about pure language choice. When you write php, you probably have to put up with apache. Maybe you also put up with using a database. It's those things that can limit your scalability.
