It is roughly correct, different neurons propagate signals at different speeds but it is in the ballpark. However, the brain is massively parallel so the slow clock is not evidence for it precomputing stuff.
We can evaluate a lot of things that are one inferential step away, because of the parallelism. But we're very bad at bridging more than a few inferential steps because parallelism is almost of no use there. It's very difficult for a human brain to think a few chess moves ahead, but incredibly easy to dismiss a thousand obviously stupid moves in a fraction of a second. The human brain has to precompute almost everything in order to function.