>> ease/speed of prototyping, pure performance, a good concurrency model > out of curiosity, which language + library/framework pairs do you think is a better tool than javascript + nodejs when evaluated along each of your 3 metrics?

ease/speed of prototyping: Ruby (or Python, even), Go, [Crystal]

pure performance: Go, Rust, C++, C, Scala, [Crystal]

a good concurrency model: Erlang, Go, [Crystal (similar to Go)]

A small note about why Crystal is in brackets: The language is very young compared to the rest, and it's currently very volatile in terms of syntax/API stability, or even compiler semantics, but it looks very promising and it's quite easy to get started with it or to become a contributor because the compiler is written in itself. http://crystal-lang.org/ (Sadly, /docs is a bit outdated and misses some crucial parts, but /api is very comprehensive).

Clojure - if you're looking for a dynamically typed language with REPL driven development process - it can leverage the huge JVM ecosystem and the concurrency tools are best in class compared to mainstream languages. Performance often comes down to idiomatic vs optimized - but you can optimize stuff and there's always "drop to Java transparently" card if you hit a wall in some area.

I choose Rails for fast prototyping, recently Go if I want pure performance that I can't get from caching, and Elixir + Phoenix if I need a good concurrency model and/or high throughput, but don't necessarily care about the microseconds that can be shaved off with Go.

I think the issue there for most is that you mentioned three different programming languages.

That's interesting.

I should do some real world benchmarks on Elixir vs Go for my own purposes.

But I'd imagine if you really care about microseconds, you should use a language without GC.

Rust would seem a good choice.