The lean startup "feedback loop" was with
customers (not
coworkers). The idea was that you iterate on your
viable product (not
vibe prototype) with the market that derives value from it.
The slow part is finding those customers, syncing your deliveries with their processes, giving them time to meaningfully assess new workflows and features in the course of their business operations, collating the feedback you receive from all of them, and merging that feedback with your organization's long term growth objectives to drive new ideas into development. Well-developed organizations layer this inescapably slow flow across numerous parallel channels so engineering utilization can stay high since healthy engineering already cycled much faster than these market-engaged flows can.
Neither coding nor internal prototypes were the slow part. Market engagement and market-informed product planning were the slow part. And still are.
You may not realize it yet, and maybe you've just misrepresented it, but most of what you seem to be describing is usually considered wheel-spinning and navel-gazing. You may have made your internal process cycle faster, but you very likely just turned a wasteful busywork churn into a more efficiently wasteful busywork churn.