[krupan]

It's not the speed of light that's the long pole in the timing constraint, it's the "switching" time of the circuits that make up the digital logic. You can't switch from zero to one or one to zero instantaneously. It's modeled (or at least it was when I went to college) as a simple RC circuit. Your clock can't tick any faster than the time it takes to charge the slowest capacitor in your system. That's why chips have gotten faster and faster over the years, we've reduced that charge/discharge time (not because we've sped up light).