It might be down to memory/space allocation after all. All hashtable requires rehashing up to certain fill rate, otherwise it will regress to linear look up. F

It might be down to memory/space allocation after all. All hashtable requires rehashing up to certain fill rate, otherwise it will regress to linear look up. For the two most popular implementations, linked-list based or open addressing based, the common strategy for rehashing is to create another instance of hash table and copy the KV over. But on disk, this will be disastrous.