express-slow-down is an Express.js middleware library that rate-limits clients by slowing down responses instead of blocking them outright. It wraps `express-rate-limit` internally, using a custom handler that computes a delay based on request count and configurable options like `delayAfter`, `delayMs`, and `maxDelayMs`. The middleware increments a per-client counter (via a store, defaulting to an in-memory store), and when the count exceeds a threshold, it delays subsequent responses. It supports custom stores, async functions for dynamic delays, and standard/legacy rate-limit headers. The codebase is well-structured TypeScript with comprehensive unit and integration tests, and follows modern ES module and TypeScript practices.