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I've read that it can't even be as fast as JS, because everything is monkey-patchable at runtime. Maybe they can optimize for that when it doesn't happen, but remains to be seen.

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I've heard similar claims but I don't think it's true.

JavaScript is just as monkey-patchable. You can reassign class methods at runtime. You can even reassign an object's prototype.

Existing Python JIT runtimes and compilers are already pretty fast.

Python is probably much more monkey patchable. Almost any monkey patching that JavaScript supports also works in Python (e.g. modifying class prototype = assigning class methods), but there are a few things that only Python can do: accessing local variables as dict, access other stack frames, modifying function bytecode, read/write closure variables, patching builtins can change how the language works (__import__, __build_class__). Many of them can make a language hard to optimize.

You can always use optimistic optimization strategies where you profile the fast path and optimize that. When someone does something slow, you tell them to stop doing it if they want better performance.

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