Nowadays most developers uses express by default to build a new Node.JS app. It's is popular, easy to use and even Nest.JS uses it behind the scenes.

But if you take a look at the Fastify docs, you’ll see that it works quite differently from Express:

• Fastify uses fast-json-stringify by Matteo Collina with built-in functions based on schemas for serialize objects too fast
• Fastify uses a radix tree for routing - a compact and optimized tree built during server setup.
• It’s minimalist, with a strong focus on core performance
• Native validation using JSON Schema via Ajv

Express working as well but for optimization...

• Express uses JSON.stringify for serialize objects
• Linear routing based on route definition
• Minimalist but depends on directly middlewares
• No native validation

Below, you can see two files with simple implementations using Express and Fastify.I also ran a quick benchmark with Autocannon, and here are the results:

Express might feel comfortable, but give Fastify a try at least once.

Ref:

https://fastify.dev/docs/v2.15.x/Documentation/Server/
https://ankitpandeycu.medium.com/unleashing-the-potential-of-radix-tree-35e6c5d3b49d
https://www.npmjs.com/package/fast-json-stringify
https://fastify.dev/benchmarks/