Beyond writing the spec down, you can share the spec or use someone else's spec. That's why spex.build was created, to be a hub with versioned specs so people can just create their own implementations, in the language, style, and particulars that they want.
Even it's slow, you can run multiple agents. You can have one doing changes, while another writes documentation, while another does security checks, while another looks for optimizations. Persist finding to markdown files to track progress and for cross-agent knowledge sharing if need. And do whatever else while it's all running. This has been my experience.