Yes, but then extensions can already release the GIL and use the simple and industrial strength std::thread, which is orders of magnitude easier to debug.
(And as a side note: I have never, in around a decade of writing C++, heard std::thread described as "easy to debug.")
I expected that dropping down to C/C++ would be a large jump in difficulty and quantity of code, but I've found it isn't, and the dev experience isn't entirely worse, as, for example, in-editor code-intelligence is rock solid and very fast in every corner of my code and the libraries I'm using.
If anyone could benefit from speeding up some Python code, I'd highly recommend installing cppyy and giving it a try.
To be honest, I don't know much about the speed, as my use-case isn't speeding up slow code.