https://en.wikipedia.org/wiki/Allocator_(C%2B%2B)

The default allocator uses operator new to allocate memory.[16] This is often implemented as a thin layer around the C heap allocation functions,[17] which are usually optimized for infrequent allocation of large memory blocks. This approach may work well with containers that mostly allocate large chunks of memory, like vector and deque.[15] However, for containers that require frequent allocations of small objects, such as map and list, using the default allocator is generally slow.[4][17] Other common problems with a malloc-based allocator include poor locality of reference,[4] and excessive memory fragmentation.

И дальше длинное описание как сделать крутой костыль, с буффером в котором хранить все не гигантские объекты. А то ой, сишечка тормозит.