The performance complaints seem to be mostly about buffer management. Rust hashers may flush their internal buffers whenever they want, as long as it's deterministic:
> Thus to produce the same hash value, Hash implementations must ensure for equivalent items that exactly the same sequence of calls is made – the same methods with the same parameters in the same order.
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[ 3.1 ms ] story [ 16.4 ms ] thread> Thus to produce the same hash value, Hash implementations must ensure for equivalent items that exactly the same sequence of calls is made – the same methods with the same parameters in the same order.
https://doc.rust-lang.org/std/hash/trait.Hasher.html