I don't think it's enough to just add JIT. For a runtime system the garbage collector is important, it's important to do optimizations such as V8's hidden class

I don't think it's enough to just add JIT. For a runtime system the garbage collector is important, it's important to do optimizations such as V8's hidden classes, function inlining, to remove the GIL etc. This is probably the reason why their perfomance gain is so small - - they have added JIT, while applying only a few smart optimizations. I don't think Unladen Swallow should be merged into CPython until the benefits are proven. Currently it will just add an extra layer of complexity, worsen Python's greedy management of memory and add little benefits in perfomance.