>>1
Once you start working on important things the usefulness of iostream really starts to decline, because the important things have a GUI, or you're working in places far awar from where the output and input are generated. About the only thing for iostream to do in programs like these is update error logs. Especially if you consider what C++ spends most of its time doing in the real world.
If you're just doing your homework or something then there is no need to listen to anyone's performance claims, but it probably doesn't matter anyway.