[EricBurnett]

You're right that the speed of light remains a constant limitation on propagation delay, but the defining limitation on the speed of computation is rather the clock speed - how long it takes for each round of computation. Electrons are comparatively slow due to the time it takes to fill and stabilize a transistor. Our hypothetical new type of computer will have to be faster to converge, rather than faster to propagate.

You're right about the bit flips though. I don't know if a gravitational wave computer is actually ever going to be feasible, just an interesting dream for the far future. Hopefully there are more options to consider in the meantime :).