Express vs Fastify

Express and Fastify are the two most popular Node.js web frameworks. Express is the incumbent with the largest ecosystem, while Fastify is the performance-focused challenger with modern developer ergonomics.

Performance Gap

Fastify is roughly 2-3x faster than Express in benchmarks. The performance comes from: schema-based serialization (fast-json-stringify), efficient routing (find-my-way), and avoidance of Express's middleware chain overhead.

For most applications, Express is fast enough. But if your service is I/O-bound and handling thousands of requests per second, Fastify's throughput advantage translates to real cost savings in infrastructure.

Plugin Architecture

Express's middleware runs in a global chain — every request passes through every middleware unless you manually scope it to routes. This becomes complex in large applications.

Fastify's plugin system provides encapsulation. Each plugin has its own scope, and decorators (custom properties on the request/reply) do not leak between plugins. This makes large applications more maintainable and testable.

Validation and Serialization

Fastify validates incoming requests and serializes responses using JSON Schema. This is both a correctness feature (malformed requests are rejected before hitting your handler) and a performance feature (schema-based serialization is faster than JSON.stringify).

System Design Relevance

When designing a REST API or API gateway in a system design interview, the framework choice is less important than the architecture. However, mentioning Fastify shows awareness of performance optimization at the framework level.

For more backend framework comparisons, see our technology guides and system design interview preparation.