... which they have done, for some definition of "best". They've chosen an implementation with good cache locality and lower memory overhead (and presumably be

... which they have done, for some definition of "best". They've chosen an implementation with good cache locality and lower memory overhead (and presumably better average-case speed) than the chaining implementation usually used for std::unordered_map. The downside is that its worst-case speed is pretty bad. On the other hand, I believe they use Siphash by default, which means it's very difficult to even intentionally trigger worst-case behavior without being able to inspect the process's memory for the random Siphash key.