Things would be a lot simpler if it were faster though :)
Consider a modern cpu running at 3ghz. It can retire some instructions on every cycle. Light can travel one mm in that time. That means the part of the core that decodes the incoming stream of insns is effectively outside the lightcone of the part that retires instructions.
A related point is that we can't just make a single CPU do more computations per unit time by making the CPU bigger because the finite speed of electricity (which is upper bounded by the speed of light), determines the largest size of the CPU before the different paths that electricity takes through the CPU logic gates become out of sync.
Then the speed of light is still not slow, cpus are just really fast. Point being, everything is relative, and lightspeed isn't necessarily the limiting factor.
