std::map is usually implemented as a Red-black tree. Which is basically a B-tree with branching factor 2. This is less cache friendly than higher branching fact

std::map is usually implemented as a Red-black tree. Which is basically a B-tree with branching factor 2. This is less cache friendly than higher branching factors, so it's entirely expected to be slower than B-trees. On the upside, it offers stable pointers which higher branching factors cannot offer.