If you know what a compiler will optimise, might optimise and can't optimise, you can get an intuition for what the generated code will be executing, but even A

If you know what a compiler will optimise, might optimise and can't optimise, you can get an intuition for what the generated code will be executing, but even ASM is not "full control" and often not super useful because what you think is happening will shuffled around again by the CPU. The execution time for the same asm will vary significantly depending on architecture. If you have a good mental model of modern CPUs, in a simple loop, you can estimate what you think the bottlneck of the function will be, either by counting the micro ops or the number of stack / heap memory reads or memory allocations, etc, to estimate what is really happening to work out how you can optimise it, otherwise you're just shooting in the dark trying random combinations of flags or code not understanding why something worked or didn't work. At least in C/C++ that model works. In slow languages like python/javascript/etc, doing simple operations doesn't translate down to the very low levels at ALL. Generally if you imagine the worst possible way you can think of for how something will execute in a simple loop and multiply it by 10, it might be close.