Question: How is hashbrown faster than the stdlib HashMap?

I'm curious as to how the hashbrown crate can have up to 2x performance on certain operations, even though it looks like the standard library's HashMap is just a wrapper for hashbrown.

I understand that a wrapper could add a small overhead, but 50% of the original performance is a bit silly, especially considering all of the functions in the wrapper are #[inline], so there should be no overhead in calling most functions.

Does anyone know the reason for this?