For JS, I know Haxe sticks to relative best practices for performance. For example, the strictly typed nature of Haxe tends to result in the best performance with Javascript JIT compilers, so Haxe code is fairly comparable to well-written Javascript. (Note, I'm talking about regular JS here, not canvas / openFL graphics stuff - obviously that would require your own benchmarks).
http://stackoverflow.com/questions/1...ode-peformance