Why Fastify achieves significantly higher throughput.

In Simple Words:

1. Radix Tree Routing: Uses find-my-way for $O(K)$ route matching where $K$ is route length.
2. Pre-Compiled JSON Serializer: Uses fast-json-stringify which compiles JSON schemas into optimized C-like string concatenators, bypassing slow JSON.stringify().
3. Optimized Object Allocations: Minimizes garbage collection pressure per request.

// fast-json-stringify example used internally by Fastify:
const fastJson = require('fast-json-stringify');
const stringify = fastJson({
  type: 'object',
  properties: { id: { type: 'integer' }, name: { type: 'string' } }
});
console.log(stringify({ id: 1, name: 'Alice' })); // 2x faster than JSON.stringify