that's obviously not true.
For "small challenge" however, i do agree that C can always be the fastest language. This comes from the fact that you understand the C compiler very well and how it can translate to assembly language.
Now look at programs that deal with complex algorithms on complex data structures. You have to understand how the compiler will optimize the generated assembly code of these complex algorithms. Now guess what? On many complex examples the assembly generated is way suboptimal. So what do you think? You can actually imagine a higher-level language that has a better compiler for this particular task. This language will lead to faster code than yours, no matter how good you are.
But does this happen in real world application? The answer is simply yes. Just have a look at the computer languages shootout and you will see that C is beaten on some tasks by *some* higher-level language. Most of the time the language is OCaml, which is a very very higher-level language than C.
Conclusion: what makes a language fast is not the language syntax or semantics, but the compiler
What is algebra, exactly? Is it one of those three-cornered things? -- J.M. Barrie