At work I use Scala and it's of course rock-solid, but it feels like a drag to develop and deploy–taking minutes to iteratively develop, and tens of minutes to test and build an assembly jar or Docker image. For a side project I'm using OCaml and greatly enjoying the fast feedback and deploy experience. For parallelization, there's not really much choice than to use multiple processes for now; but in the near future true multicore support will be shipped. Performance is very reasonable–most of my responses are in low milliseconds. Take a look at https://aantron.github.io/dream/