Check out the link that he is responding to in his essay (why should I ever use C again). There's an entertaining argument about speed in there - (I'm paraphrasing here)... If speed is so important, why aren't you writing assembly? The mostly likely answer is that slower C is better than fast assembly. But then why isn't slower <your fave> better than fast C?

The logic isn't 100% airtight - it is possible to argue that benefits of C are sufficient to take the performance hit over assembly, but not so great to take the hit from C to Ruby.