> Python is harder to JIT because of the way __dict__ works.

Heh, how about how `__class__` works? Not that I recommend actually doing this, but you can literally change the type of an already instantiated object by reassigning `__class__`:

    >>> class Dog:
    ...     def hi(self):
    ...             print("woof")
    ...
    >>> class Cat:
    ...     def hi(self):
    ...             print("meow")
    ...
    >>> dog = Dog()
    >>> dog.hi()
    woof
    >>> dog.__class__ = Cat
    >>> dog.hi()
    meow
    >>> isinstance(dog, Dog)
    False
    >>> isinstance(dog, Cat)
    True
    >>>

I've never bought this argument. Underneath all of the magic inheritance in Python is just the MRO (method resolution order). That's not characteristically any different from JavaScript's prototype chain. There's no reason a Python JIT couldn't optimize everything JavaScript does, and if you do specifically access `self.__dict__`, that can be a cold, unoptimizable path.

Same as for the common lisp or raku mop. You check for the default, and use the fast path if so. And if you can guarantee that nobody changed those keys or values, lift that check to the front.

See Self.