I think Ruby solutions are the best. You can generate API-only application by just adding the --api filter to the rails new command. Sinatra is also a good option for APIs. Ruby itself is the most expressive language. With the release of version 2.3.X (and above) Ruby improved its performance a lot, combining this with Puma will give you a nice performance and concurrency combined with the maturity of Rails (or Sinatra).
If you're looking for multiple times better performance than Ruby (or Node) but still a codebase which is highly manageable and simple, then go for Golang.