By default, cin/cout waste time synchronizing themselves with the C library’s stdio buffers, so that you can freely intermix calls to scanf/printf with operations on cin/cout. Turn this off with std::ios_base::sync_with_stdio(false);
Many C++ tutorials tell you to write cout << endl instead of cout << '\n'. But endl is actually slower because it forces a flush, which is usually unnecessary. (You’d need to flush right before prompting the user for input from cin, but not when writing a million lines of output.) Write '\n' instead of endl.
There was a bug in very old versions of GCC (pre-2004) that significantly slowed down C++ iostreams. Don’t use ancient compilers.
Avoid these pitfalls, and cin/cout will be just as fast as scanf/printf. My benchmarks indicate that cin/cout is actually 5%–10% faster.