Hmm, I'll have to look for benchmarks then. My general experience is that it is hard to outsmart the VM performance wise in the long run

We're not competing with VMs here. From what I hear, like many other built-ins, "native" promises in every VM are implemented in JavaScript. They're not always well done (V8 promises were really bad for a while), and pay a mandatory overhead for useless "privacy" features dictated by the spec.