It is obviously implementation dependant. C++ does a bit of extra work - it performs code conversions where required (std::codecvt) and it provides robust handling of stream errors (by guarding them with sentries).
There is no technical reason why one should be noticeably slower than the other. You can execute a few million instructions in the time it takes the user to type in one character.
I am fairly certain that this particular author would have also raised another question - one that Stroustrup answers in his FAQ:
Why is the code generated for the "Hello world" program ten times larger for C++ than for C?
It isn't on my machine, and it shouldn't be on yours. I have even seen the C++ version of the "hello world" program smaller than the C version. In 2004, I tested using gcc -o2 on a Unix and the two versions (iostreams and stdio) yielded identical sizes. There is no language reason why the one version should be larger than the other. It is all an issue on how an implementor organizes the standard libraries (e.g. static linking vs. dynamic linking, locale support by default vs. locale support enabled through and option, etc.). If one version is significantly larger than the other, report the problem to the implementor of the larger.