Why is Function.prototype.bind slow?

Based on http://jsperf.com/bind-vs-emulate/6, which adds the es5-shim version for comparison, it looks like the culprit is the extra branch and instanceof that the bound version has to perform to test if it’s being called as a constructor.

Each time the bound version is run, the code that gets executed is essentially:

if (this instanceof bound) {
    // Never reached, but the `instanceof` check and branch presumably has a cost
} else {
    return target.apply(
     that,
     args.concat(slice.call(arguments))
    );

    // args is [] in your case.
    // So the cost is:
    // * Converting (empty) Arguments object to (empty) array.
    // * Concating two empty arrays.
}

In the V8 source code, this check appears (inside boundFunction) as

if (%_IsConstructCall()) {
    return %NewObjectFromBound(boundFunction);
}

(Plaintext link to v8natives.js for when Google Code Search dies.)

It is a bit puzzling that, for Chrome 16 at least, the es5-shim version is still faster than the native version. And that other browsers have rather varying results for es5-shim vs. native. Speculation: maybe %_IsConstructCall() is even slower than this instanceof bound, perhaps due to crossing native/JS code boundaries. And perhaps other browsers have a much faster way of checking for a [[Construct]] call.