std::unordered_map is supposedly slow because it has fairly stringent iterator invalidation requirements. In my experience, unless you wring the most performance out of your code as you can, it's not a huge issue; it's generally faster than most casual implementations.
Yeah people here are dumping on it for being non-performance-optimal while ignoring that higher performing versions have restrictions and design limitations. unordered_map is fast enough for probably 98% of uses, and the other 2% may be better off finding a specialized version best for their particular case.
