Also, just think about all the optimizations your "serial" programming language does -- are your yourself really write all those mutations? Or is that the compiler, that in many cases can do a much better job? Now what if the language's semantics allowed even more freedom for the compiler in exchange for more restrictions on the language? Sure, we still don't have "sufficiently advanced compilers" that would replace programmers, but FP languages absolutely trade blows with most imperative languages in many different kinds of problems. Very visibly when parallelism comes to the picture, as it turns out, a smart parallel-aware data structure will easily beat out their serial counterparts here.
