v8 Engine - Why is calling native code from JS so expensive?

Based on multiple answers to other questions, calling native C++ from Javascript is expensive.

I checked myself with the node module "benchmark" and came to the same conclusion.

A simple JS function can get ~90 000 000 calls directly, when calling a C++ function I can get a maximum of about 25 000 000 calls. That in itself is not that bad.

But when adding the creation of an object the JS still is about 70 000 000 calls/sec, but the native version suffers dramatically and goes down to about 2 000 000.

I assume this has todo with the dynamic nature of how the v8 engine works, and that it compiles the JS code to byte code.

But what keeps them from implementing the same optimizations for the C++ code? (or at least calling / insight into what would help there)

via Thomas Rosenstein