It's a very old story: Hand coded assembler can commonly get a factor of several faster execution time on compiled code and more when using special instruction

It's a very old story: Hand coded assembler can commonly get a factor of several faster execution time on compiled code and more when using special instructions on special data. But on large programs, no, because some of what compilers and runtime libraries do for large programs is too difficult for hand coded assembler. Indeed, some of what compilers do in register allocation (and maybe cache usage) will be too difficult for hand coded assembler. More generally, now the execution time of a small, single threaded program is nearly never of much concern. E.g., when have a processor with dozens of cores, it's easy to let a small, single threaded program have a core and then f'get about it. Execution time remains important but now mostly just for large, multi-threaded programs.