Speed is not a matter of choosing a certain language, speed is a matter of choosing the right algorithms. Apart from that, writing whole programs in assembler is not recommended on modern operating systems because you would have to program against a C-level interface. But nothing (except for portability) speaks against writing small functions (e.g. for image processing) in assembler. But don't get the illusion that asm is faster than C. Most C-compilers nowadays produce optimized asm code that is faster than anything you would write yourself in asm.