The problem is (perhaps amusingly) with refcounting. As the processes run, they'll each be doing refcount operations on the same module/class/function/etc objects which causes the memory to be unshared.
Only where there is memory churn. If you’re in a tight processing loop (checksumming a file? Reading data in and computing something from it?) then the majority of objects are never referenced or dereferenced from the baseline.
Also, since the copy on write generally is memory page by memory page, even if you were doing a lot of that, if most of those ref counts are in a small number of pages, it’s not likely to really change much.
It would be good to get real numbers here of course. I couldn’t find anyone obviously complaining about obvious issues with it in Python after a cursory search though.
