I think JS can really zoom if you let it. Hamsters.js, GPU.js, taichi.js, ndarray, arquero, S.js, are all solid foundations for doing things really efficiently. Sure, not 'native' performance or on the compile side, but having their computational models in mind can really let you work around the language's limitations.
JS can be pretty fast if you let it, but the problem is the fastest path is extremely unergonomic. If you always take the fastest possible path you end up more or less writing asm.js by hand, or a worse version of C that doesn't even have proper structs.
I find these userland libraries particularly effective, because you'll never leave JS land, conveniently abstracting over Workers, WebGL/WebGPU and WASM.
