That said, one thing I've heard a lot from NativeScript TSC members building on top of QuickJS is that QuickJS has staggeringly low overhead for calling C functions. In fact, I was shown benchmarks in which it calls native functions faster than JS functions (perhaps it's easier to optimise because JS functions have complications like closure capture to worry about). I imagine this has conceptual overlap with how V8's Fast API works.
We had some performance issues with marshalling and unmarshalling, but that was mostly due to API design and how our codebase was structured. As per usual, optimizing scripts was the best bang for buck.