Isn't it the opposite? cgo calls always have to switch to a larger stack (because there's no way to know in advance how much stack a function will use, and C f

Isn't it the opposite? cgo calls always have to switch to a larger stack (because there's no way to know in advance how much stack a function will use, and C functions don't support dynamic stack switching like Go functions do), so they have some fixed overhead per call. And that fixed overhead is most significant in relative terms when the call returns quickly.