The reason the custom hashtable wins out isn't something generally applicable. For the very specific dataset used in the challenge, the hash function could be radically simplified, to just a single multiply and rotate left.

To be fair, I didn't try out the hashbrown API. Maybe that, together with FxHash, would have been enough for the official dataset with 97% of keys < 16 bytes. But, I was optimizing for the "10k" dataset as well, with a lot of longer keys, and hashing just the first 8 bytes was the winning idea.