asm.js code is "just" JS code in the sense that it is a subset of JS: a non-asm.js-aware JS implementation can treat it the same way as normal JS, and it will execute correctly. But in implementations where it's fast, asm.js is usually handled separately.

It looks like Mozilla's asm.js implementation used to have the exact same problem: https://hacks.mozilla.org/2015/03/asm-speedups-everywhere/#c... I'm not familiar enough with asm.js to know if that's still a problem.