You are correct, in theory. AVR chips used in most Arduino don't have hardware supported multiplication I think, and almost certainly not division, and definitely not floating point operations of any sort. Even more sophisticated chips, that can do such things in hardware, can't do then in a single cycle, so multiplying by -1 will always be slower than subtracting from 0 on almost any chip, I suspect. But, in practice, as you suspect, it may be that the compiler can spot and optimise stuff like that and do it in a more efficient way so that it makes no difference.