I think in case of CPython it's just Python being slow to do anything. There are costs of the interpreter, GIL, and conversion between Python's objects and low-level data representation, but the FFI boundary itself is just a trivial function call.

> but the FFI boundary itself is just a trivial function call.

Which no different than Go, or any other language under the sun. There is no way to call a C function other than trivially, as you put it. The overhead in both Python and Go is in doing all the things you have to do in order to get to that point.

A small handful of languages/implementations are designed to be like C so that they don't have to do all that preparation in order to call a C function. The earlier comment included CPython in them. But the question questioned how that is being pull off, as that isn't the default. By default, CPython carries tremendous overhead to call a C function — way more than Go.