Looking around the forum (prompted by vague memories of seeing such things), it does look like there have been a few such projects.
Here for example is one:
I haven’t used it myself, nor heard much, so I can’t recommend for or against it, but it does appear to be one such project!
There’s also this thread regarding the use of a third-party ECS system:
I believe that this is pretty much accurate, yes.
In short, the parts that use Python (in particular your own Python code, but also a few other components) will (naturally) run as Python runs. But with most of the engine written in C++, most of the engine should indeed run as C++ runs.
(Indeed, I’ve seen it recommended that, if one finds particular parts of one’s Python code running over-slowly, one can re-write the same in C++, incorporate it, and potentially get a performance boost for those parts.)