What's happening is that the std::unordered_map was a mistake. Many parts of it's API prevent implementing it efficiently (For example, every insertion requires allocation, even if you reserve). It's widely regarded, within and outside the standards committee, as a very unfortunate mistake.

swisstable (absl::flat_hash_map) is a popular choice right now for the hash table to use for C++ code that cares about performance. There are other choices as well (dense_hash_map, F14FastMap, etc), But honestly most choices these days are worse than std::unordered_map (well, boost::unordered_map is probably just as bad, since std's implementation was taken wholesale from boost without any thought!)