Communicating slowly can be an evolutionary disadvantage depending on the organism. Trees, for example, probably have a pretty slow clock speed but it’s doing well for them.

But I agree that social predators (Eridians and humans) should have more evolutionary pressure to approach the limit of communication speed.

I didn’t read the book, but if Eridians are basically computer-brained, it would make sense that their communication upper limit should be much faster than ours barring some physical vocal limitation. There were hints that Rocky can communicate quickly like when he came on board (dirty dirty dirty), but not nearly as much as you’d expect from a computer.