Theoretical:
Hand coded C will usually be 'faster' than C++. But with a good compiler, and well written code, C++ can come to par with C performance.
Practical:
With a programmer (good, bad, whatever), and a really large project, C++ can easily be FASTER than the C version. In C, it's hard to support things like std::vectors which reallocate twice the container size for optimized push_back()s, among other things, without reusing code, and making the program many times larger than it needs to be.
