To this day I believed, that std::endl is always better that \n in C++ streams (portability, for instance). It turns out that no.

A code with std::endl:

#include <string>
#include <iostream>

int main() {
  for (int i = 0; i < 1000000; ++i) {
    std::string s(1, 'x');
    std::cout << s << std::endl;
  }
  return 0;
}

Compile and run:

clang++ -o endl -O3 endl.cpp && time ./endl >rubbish

real    0m4.518s
user    0m1.080s
sys 0m3.311s

A code with \n:

#include <string>
#include <iostream>

int main() {
  for (int i = 0; i < 1000000; ++i) {
    std::string s(1, 'x');
    std::cout << s << '\n';
  }
  return 0;
}

Compile and run:

clang++ -o endl -O3 endl.cpp && time ./endl >rubbish

real    0m0.263s
user    0m0.236s
sys 0m0.008s    

The difference is obvious. The second one is much faster.

std::endl always flushes the stream. In turn, \n simply puts a new line character to the stream, and in most cases this is exactly what we need. And std::flush, if necessary, can be called afterwards, once, explicitly.