How so? Sharing arbitrary objects via memory requires some form of coordinated memory management. So you still need a process-wide GC, meaning you're probably stuck with that 30% hit from thread-safe refcounting.

And all the other locking issues are still in play. How would you share a list with another interpreter such that they don't both have access to the same list items? Any solution that looks like a deep copy isn't going to be much better than just serializing across a process boundary.