Note that just because someone's invented transistors that can switch at 300 GHz, doesn't mean that you can make a processor which will be able to run with a 300 GHz clock speed. These are two completely different issues, and articles with things like "300 GHZ!" in the headlines often play off this confusion to get page views.
There are already transistors which can switch at much higher frequencies. They're mainly used in radio communications. The promise of graphene semiconductors is that they may be a way of replacing or supplementing silicon, so it's nice that graphene transistors are turning out to have good performance, but the really big issue continues to be figuring out how to mass-produce non-trivial devices. Or even trivial ones.
How would it be... impractical? That means light goes ~ 7,000 times further than the width of the gate (140 nanometer ones have been made) in the same time span. They're hardly pushing any theoretical limits as pertain to the speed of light.
(light speed / 300 billion hertz) * (1 billion nanometers per meter / 140nm) = 7,137.91567 , according to Google's calculator.
It's a problem because in chip therms that is tiny, and you still need to interface to the outside world, using regular wiring you're much slower than 300GHz, and using optics you have a pretty serious interfacing issue.
Of course even a 10-fold practical increase would be an amazing thing, but I just can't see the 300GHz ever becoming a reality given the constraints that physics imposes on this.
Sorry, you are right. The more complex your chip is going to be, the more trouble you have with RF issues.
Nevertheless, what I meant is, there will be components working in the low THz range, as they are already demonstration objects. Most probably you won't see this components in consumer products or in computing, due to cost, scaling and implementation issues. But for some applications I think this will be applicable.
Every wire is not 'just' a wire it is also a capacitor and a coil as well as a resistor.
The parasitic LCR filter created by even the shortest stretch of circuitry is going to have a cut-off frequency well below the frequencies quoted here.
The 'C' component is further increased by the capacitance of the input on the other end, and charging that capacitor is slowed down further by any leakage there (another R).
There are already transistors which can switch at much higher frequencies. They're mainly used in radio communications. The promise of graphene semiconductors is that they may be a way of replacing or supplementing silicon, so it's nice that graphene transistors are turning out to have good performance, but the really big issue continues to be figuring out how to mass-produce non-trivial devices. Or even trivial ones.