Subinterpreters share a common GIL. Also subinterpreters don't share Python objects, which means among other things that all modules are imported separately in each SI, which increases startup time, memory usage and reduce cache effectiveness.

It's a band-aid. If you want to run Python code in parallel, without large overhead, then CPython is simply not your environment to do so, and Python is not a good choice overall in that kind of endeavour.