I am writing an interpreter for a functional programming language that is going to run in the browser. Needless to say, performance is the one and only concern in this application. Emscripten is an LLVM→JavaScript compiler which claims to produce programs that run only 2~3 times slower than native. It is really surprising, considering actual JavaScript applications tend to run 7 to 10 times slower than native, according to the Computer Benchmarks Game. And those are exactly the types of application I am worried about.
So, said that, would it be profitable to code my application in C, and then compile it to JavaScript using asm.js, or is there something I am missing?