Actually, it is the merit of the language. C - and FORTRAN for that matter - are fast because their restrictions force you to program a certain way. A way that just-so-happens to be efficient (if not particularly safe).

For example, generally, people use a lot of static arrays in C. Arrays are efficient. The way you program with fix-sized structures like arrays tends to be different to how you program with dynamically-sized structures and that style of programming tends to be more efficient.

In the vast majority of cases, a C or FORTRAN program will be more efficient than other languages not because they are C or FORTRAN, but because they force you to use an efficient paradigm.

Even other compiled languages like C++ or D are, more often than not, slower than the C counterpart UNLESS they are programmed in a "C style".

e: To be clear, the fact that they are compiled low-level languages obviously does help, but I'm trying to make the point that it's not the only reason.