I feel like this is dramatically missing the point. It is trivial to come up with a communication system where signals travel at the speed of light. In fact, anything visual meets this criteria: sign language, semaphores, clicking your flashlight on and off. Radio waves travel at the speed of light. All of humanity became a giant "lightspeed-intelligent" brain when radio was first invented.
It really does matter what you're doing with those signals, how much information each contains, how many you're sending, how much power it takes to send and receive them, how they're encoded, etc. Focusing on the fact that they travel at the speed of light is silly.
> The advantage of this over biological computation is clear: Biological computation happens at max 100m/s, 6 orders of magnitude less than the speed of light
You are trying to compare computation power by measuring distances. You are basically saying "one biological computation" is a million times slower than "one silicon computation" because of how fast signals travel, completely ignoring what is actually happening in those extremely different computations. It's still not clear that brains can be compared to computers at all, but if you try to simplify it down to FLOPS (a much better measure of computation speed than "how fast do some signals go"), our best estimates are that one brain has the computational equivalent of somewhere between 1,000 and 100,000 modern GPUs.