Light-4j framework is one of the fastest API frameworks on the market and there are a lot of effort to make that happen. There are a lot of frameworks, languages to build microservices but most popular ones are Java, Go and Nodejs.
Nodejs used to be really fast compare with Java EE platforms but it has a lot of issues in enterprise world.
Go has potentials but is still not mature in enterprise applications. We are starting with Java due to the fact that it is highly optimized and there are so many existing investments in most of organizations.
We have tested most of microservices frameworks and here is the benchmarks along with source code.
All the implementations for TechEmpower are following RESTful style although there are a lot of middleware handlers are removed. The source code can be used as an example on how to implement high performance microservices.