A light-nanosecond is roughly 30cm (or roughly a foot, if you'd like). So at 1Ghz, light in a vacuum can reach 30cm per tick. Electrical signals in copper travel at around (1/3)c can reach 10cm.
(I find it very funny that we can use such nice numbers, given that both the meter and second were defined long before c was known.)
So at 20Ghz, this is cut to 5mm. In other words, your die can have a diameter of at most 5mm, no matter what crazy architecture you can imagine, if you want the processor to function at 20Ghz. So in a space of (5mm)^2, you're sending 20 000 000 000 signals per second, which need to be much stronger than background noise.