Yeah the whole reason asm.js is fast is because it strips out the performance hostile parts of js.
The ReactVR framework can use asm.js to gain acceleration but you as a front-end developer could stick to best JS VR practices to not get in the way of the performance ReactVR with asm.js brings you (if it uses it now or in the future).
Any language that uses LLVM isn't using the language either, by this definition. That would be a shock to most C++, CUDA and Swift developers.
How so? LLVM is used to implement language features, performance-hostile ones included. asm.js is a codified way to avoid language features.
You may be confusing layers here. asm.js as a compilation target is used the same way as LLVM- it is used to implement the source language's features, while simultaneously avoiding some of JS's.
I'm sorry, I didn't think I needed to spell it out: LLVM front-end compiles a high-level language into an intermediate representation ("IR") which is then transformed by the backend into machine code. A compiler targeting wasm occupies the same space as the LLVM front-end. Now, in a browser there is not as obvious a back-end, but it's there: it's the JSVM JIT code.
But then you're no longer using the language. The performance-hostile semantics are still there, you're just not using that part of the language, and thus losing out on all their benefits.