The unique fitness of nature for carbon-based life and intelligent beings of our biology is an empirical discovery.
Surely there could not be an equivalent ensemble of fitness in nature for some other type of life.
One area where very fast nerve conduction is vital is vision, more specifically, in keeping the eyes fixed on some object in the field of vision while in motion.
As every medical student comes to learn when first dissecting the human body at medical school, our limbs are almost entirely composed of muscles.
It is likely that no further improvement in muscle power can be achieved by increasing the density of packing of the myosin motors.