Unfortunately something like 0.7c is about the fastest speed of EM wave propagation in an optical waveguide or along a copper waveguide. Another comment here gives a slightly faster example with n=1.3, which is maybe achievable in some kind of polymer. Or in highly purified water, for what it's worth.
You can get a mild speedup, 40% or something, by moving to free space. But that is an unbelievable can of worms, taking all the signals out of the waveguides and somehow still getting 1B signals going to the right place. The 40% speedup doesn't remotely pay for giving up solid state waveguides.
"Dispersive" fortunately doesn't mean a meaningful slowdown. It just means that a transmitted bit will travel at a range of slightly different speeds. If it goes very far, the shape of the pulse will get messed up. But that's a problem people are already pretty good at solving.
If n=1.3 -> v = c/1.3 = 77% of c
Where some are said to reach up to 99.x % the speed of light in a vacuum.
Most fascinating thing i've read years ago they'd be the prime candidate for manufacturing in space, because real vacuum.
Have you ever tried wiring any non-trivial logic without flip-flops? Say, a simple signal routing layer. Even the most basic bits of logic becomes much less efficient to downright impossible without storage.
I don't see how the wavelength is comparable to transistor size because as you switch to the optical realm, the benefit of information propagation at speeds near c (or c, if you're pulling a vacuum) means physical size doesn't matter as much. At 4Ghz you can move information 7.5cm in one cycle, and that's a pretty large distance compared to any integrated circuit I've ever seen.
Why is storage necessary? If you can move bits to optical gates and get a result back it seems to me like you can work around the fact that, in an electrical system, capacitance and heat (due to density achieved in the quest for minimizing capacitance) start to limit the computation you can do.