Well it is a very simple solver, you can't change it after it's synthesized and it's not true it uses no power - you need strong enough incoming light. That's your power source. It's probably safe to assume that each layer removes some power in a mutiplicative fashion, therefore it's only so much you can do in a single glass step before the beam dissipates.

And as far as timing goes - 1 cm of glass delays by ~50 picoseconds. That corresponds to around 20 gigahertz. Fast, but not mind blowingly fast.