Removing the GIL requires operations that are protected by it when it exists to be made thread safe in ways which they don't need to be with it, which has some overhead. Even having multiple options from which the situationally correct one (e.g., a lighter-weight one in guaranteed single-threaded or GIL active cases) can automatically be selected has some overhead. (Conceptually, you could have separate implementations that can be selected by the programmer with zero runtime overhead where not needed, but that also has more conceptual/developer overhead, so other than particular things where the runtime cost is found to really bite in practice, that's probably not going to be a common approach for core libraries that might need to be called either way. There's no free lunch here.