There's actually no overhead of "still using Python" because we don't use Python. The overhead in Function exists solely because we have a bunch of sugar added to create a unified interface for calling different kinds of functions (i.e. generator functions, etc).

You don't have to take my word for it: you can pull the C++ source code that we generate from a given Python function and inspect it yourself: https://docs.fxn.ai/predictors/compiler#compiling-binaries