Why is a naive `iter_swap`potentially much slower than `swap`?

From David Abrahams and Aleksey Curtovoy's book "C++ Template Metaprogramming", I learned that iter_swap (see below) would be much slower than std::swap sometimes. Although the book has some explanation, I did not quite get it, could someone explains the reason behind it with more details.

template <typename ForwardIt1>
void iter_swap(ForwardIt1 it1, ForwardIt1 it2){
  typedef typename std::iterator_traits<ForwardIt1>::value_type T;
  T tmp = *it1;
  *it1 = *it2;
  *it2 = tmp;
}

template <typename ForwardIt1>
void swap_wrapper(ForwardIt1 it1, ForwardIt1 it2){
  std::swap(*it1, *it2);
}

By applying them on std::list<std::vector<std::string>>::iterator, I found the first is 10X slower than the second even when the size of the vector (whose elements are all small strings, length less than 10) is just 10.