The specifications were vague because they were in prose, rather than a formal specification language, and written in an IDE with instant feedback, auto-complet

The specifications were vague because they were in prose, rather than a formal specification language, and written in an IDE with instant feedback, auto-complete, history and documentation. It took several days to turn around because it took a developer a half day to write the controller code and specs, to debug it, to write the PR and get it approved, to merge the branch and wait for it to be deployed. This half a day was interleaved with a full schedule as available, so routinely took a week. With a formal, executable specification that is a GraphQL query, all of the above goes away. The back-end devs don't even knows about it unless there's a problem.