Slightly tangential but c++ hash tables aren't even the fastest. The requirements on iterator invalidation force the use of extra allocations and indirections for separate chaining.
I've hand-rolled and/or reused hash tables that use various probing methods (removing both allocations and extra indirections) and the performance improvement is non-trivial (for my workloads anyhow).
I've been surprised by how slow the C++ standard data structures are (likely due to the need to support a lot of functionality). I once wrote a basic heap in C and tried benchmarking its performance at -O3 relative to the C++ STL heap. Was expecting mine to be slower compared to the highly optimized STL heap, but was wondering how much slower. Instead, it turned out to be 1.5-2x as fast (and this was despite freeing up memory when the underlying vector shrunk - something which the C++ vector does not do).
I've hand-rolled and/or reused hash tables that use various probing methods (removing both allocations and extra indirections) and the performance improvement is non-trivial (for my workloads anyhow).