Why mutex (std::mutex) is heavy?

Quite often on this site, other forums I read phrases like "mutex is heavy, better use something else". But I can't really find explanation why it's heavy? Also, if we are talking about standard C++11 before C++20, we basically have only std::mutex, used with locks or condition_variable, to make something thread-safe, I expected something from std be quite efficient, especially if it's the only tool(before C++20) to make some task, thread-safety in this case. So why mutexes and particularly std::mutex is heavy? And what we as C++ developers should use instead? Something from boost?