That's not why. Template processing happens during processing of the translation unit, so it would be expensive even if you only had a single TU in your build.

That's not why. Template processing happens during processing of the translation unit, so it would be expensive even if you only had a single TU in your build. It's true that a template has to be reprocessed for each instantiation, however, that's not merely from one TU to the next, but even inside each TU! For every distinct value of T for std::vector<T> in a single TU, the compiler has to process and generate std::vector entirely