As I understand it, even for a jit compiler, the type annotations are typically less useful than runtime behaviour. For example, at runtime, a type might typically be narrower than the compile time declaration. Also, with most gradually typed languages, the types aren't enforced at runtime, and the type system can often be circumvented, which means basing optimisations on the declared type hints will send the optimiser down useless rabbit holes.

That said, I agree with the rest of your comment - static types for correctness and static types for performance are two different goals, and most gradually typed systems work towards the former and not the matter. (And more to the point: adding types alone is not sufficient for the latter. Python is not slow because it has no types, Python is slow because it isn't designed to be fast, and changing that now would require a lot of fundamental changes to the language and ecosystem, of which types would be one of the least relevant.)