Express vs Fastify: Performance Deep Dive

Fastify wins on requests per second, by design rather than by tuning. Fastify compiles JSON Schema into a purpose-built serialiser for each route, so responses skip generic JSON.stringify reflection, and it routes with a radix tree instead of walking a middleware chain per request. Express evaluates its middleware stack for every request and serialises generically. Express wins in practice for a low-traffic service already built on Express-only middleware, where the migration cost is real and the per-request saving is not the bottleneck.

Migration complexity depends on how deeply integrated your project is. Surface-level usage can be migrated in days. Deep integrations with custom configurations may take weeks. Both communities offer migration guides. Consider running both in parallel during transition to ensure no regressions.

Both are popular with startups. Express is often chosen for its mature ecosystem, while Fastify appeals to startups that value modern architecture. The best choice depends on your team's experience and your specific backend requirements.