some of the STL is easy to improve on. For example, std::unordered_map performs poorly due to pointer stability requirements in the standard. Most performance sensitive C++ codebases will use something like abseil's hash maps instead.

Just a heads-up: if you're already using boost, boost::unordered now has open addressing containers (unordered_flat_map and unordered_flat_map) and they are among the fastest.