Efficiency of C-String vs C++Strings

Question

C++ Primer says

For most applications, in addition to being safer, it is also more efficient to use library strings rather then C-style strings

Answer 1

Well, an obvious and simple thing how they could be practically more efficient (regarding runtime) is, that they store the string's length along with the data (or at least their size method has to be O(1), which says practically the same).

So whenever you would need to find the NUL character in a C string (and thus walk the whole string once) you can just get the size in constant time. And this happens quite a lot, e.g. when copying or concatenating strings and thus allocating a new one beforehand, whose size you need to know.

But I don't know if this is what the author meant or if it makes a huge difference in practice, but it still is a valid point.