The two calls have different meanings that at the core have nothing to do with performance, the fact that it speeds up the execution time is (or might be) just a side effect. You should understand what each of them does and not blindly include them in every program because they look like an optimization.
This disables the synchronization between the C and C++ standard streams. By default all standard streams are synchronized, which in practice allows you to mix C and C++ style I/O and get sensible and expected results. If you disable the synchronization then C++ streams are allowed to have their own independent buffers, which makes mixing C and C++ style I/O an adventure.
Also keep in mind that synchronized C++ streams are thread-safe (output from different threads may interleave, but you get no data races).
cout. Tied streams ensure that one stream is flushed automatically before each I/O operation on the other stream.
cin is tied to
cout to ensure a sensible user interaction. For example:
std::cout << "Enter name:"; std::cin >> name;
cout are tied, you’re sure that the output is flushed (e.g. visible on the console) before the program expects input from the user. If you untie the streams, the program might block waiting for the user to enter his name but the “Enter name” message is not yet visible (because
cout is buffered by default, output is flushed/displayed on the console only on demand or when the buffer is full).
So if you untie
cout you must make sure to manually flush
cout every time you want something to really be displayed before expecting input on
In conclusion, know what each of them does, understand the consequences and then decide if you really want or need the possible side effect of speed improvement.