You can build an application in Ruby, deploy it onto 15 machines, and it will outperform the same application written in C and deployed to only one machine. Performance is more of a function of the underlying hardware than the language used to build it.
If a language is 30x less efficient than another language then you would likely need 30x more servers. Many folk are simply not prepared to spend 30x more than they need to on hardware. It's the difference between 20 servers and 600 servers.
Case in point: "How We Went from 30 Servers to 2":
That's only true when you are using the full resources of one or more servers. If you are only using 1/100th of the server's resources, then being 30x less efficient still doesn't require any more servers.
That doesn't negate the point though, language performance matters at certain scales.
If you split a single app instance onto 15 machines, you will lose some efficiency due to network communication unless those 15 instances can work isolated without any shared data (sessions). That may not be much but worst case: you have to write that inter-machine sync code.
