I can't really speak about Java, but for C++, std::unordered_map (which uses separate chaining) gets destroyed (performance-wise) by any number of alternate has

I can't really speak about Java, but for C++, std::unordered_map (which uses separate chaining) gets destroyed (performance-wise) by any number of alternate hash map implementations that use open addressing. There's just no question that open addressing is inherently faster for most use cases. To be fair, unlike std::unordered_map, the open addressing implementations don't come with a guarantee that they won't move the items in the hash table. But in practice that doesn't really matter anyway, since you could always use unordered_map<unique_ptr<T>> instead of unordered_map<T>. But, I digress..