C++ can actually be faster because it doesn't throw away as much type information. A good, typed C++ quicksort implementation, for example, will be faster than a C version using void*.
One of the reasons for the performance difference is that you can't inline into the libc DLL blob: If you use qsort(), you'll get a lot of overhead from calling the comparison function.
If you use a custom quicksort implementation and put it into the same translation unit as the comparison function (or compile statically and use link-time optimizations with a sufficiently advanced compiler) you can get the same performance out of C.
