It wasn't the speed of light, it was the size of atoms that was the issue here. As old-style scaling (the kind used up until about 2003) continued, leakage power was increasing rapidly because charge carriers (electrons / holes) would tunnel through gates (I'm simplifying a bit here, other bad effects were also a factor). It was no longer possible to keep increasing clock frequency while scaling down feature size. Further reduction without exploding the power requirement meant that the clock frequency had to be left the same and transistors needed to change shape.