Haskell!

If you aren't proficient in a functional language like Haskell then you haven't completed your programming education. Go, Rust, D, etc. are all just C with fancy features. Change your whole programming paradigm.

Here's a great deep-dive into Haskell web server performance: https://www.aosabook.org/en/posa/warp.html

And if you're building an API you don't need to code! Just throw postgREST in front of a postgreSQL database and get a haskell-powered API for free. No need to write a web server.